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[unleashed.git] / usr / src / uts / intel / ia32 / os / fpu.c

/*
 * CDDL HEADER START
 *
 * The contents of this file are subject to the terms of the
 * Common Development and Distribution License (the "License").
 * You may not use this file except in compliance with the License.
 *
 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
 * or http://www.opensolaris.org/os/licensing.
 * See the License for the specific language governing permissions
 * and limitations under the License.
 *
 * When distributing Covered Code, include this CDDL HEADER in each
 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
 * If applicable, add the following below this CDDL HEADER, with the
 * fields enclosed by brackets "[]" replaced with your own identifying
 * information: Portions Copyright [yyyy] [name of copyright owner]
 *
 * CDDL HEADER END
 */
/*
 * Copyright (c) 1992, 2010, Oracle and/or its affiliates. All rights reserved.
 */

/*      Copyright (c) 1990, 1991 UNIX System Laboratories, Inc. */
/*      Copyright (c) 1984, 1986, 1987, 1988, 1989, 1990 AT&T   */
/*              All Rights Reserved                             */

/*      Copyright (c) 1987, 1988 Microsoft Corporation          */
/*              All Rights Reserved                             */

/*
 * Copyright (c) 2009, Intel Corporation.
 * All rights reserved.
 */

#include <sys/types.h>
#include <sys/param.h>
#include <sys/signal.h>
#include <sys/regset.h>
#include <sys/privregs.h>
#include <sys/psw.h>
#include <sys/trap.h>
#include <sys/fault.h>
#include <sys/systm.h>
#include <sys/user.h>
#include <sys/file.h>
#include <sys/proc.h>
#include <sys/pcb.h>
#include <sys/lwp.h>
#include <sys/cpuvar.h>
#include <sys/thread.h>
#include <sys/disp.h>
#include <sys/fp.h>
#include <sys/siginfo.h>
#include <sys/archsystm.h>
#include <sys/kmem.h>
#include <sys/debug.h>
#include <sys/x86_archext.h>
#include <sys/sysmacros.h>
#include <sys/cmn_err.h>

/* Legacy fxsave layout + xsave header + ymm */
#define AVX_XSAVE_SIZE          (512 + 64 + 256)

/*CSTYLED*/
#pragma align 16 (sse_initial)

/*
 * Initial kfpu state for SSE/SSE2 used by fpinit()
 */
const struct fxsave_state sse_initial = {
        FPU_CW_INIT,    /* fx_fcw */
        0,              /* fx_fsw */
        0,              /* fx_fctw */
        0,              /* fx_fop */
#if defined(__amd64)
        0,              /* fx_rip */
        0,              /* fx_rdp */
#else
        0,              /* fx_eip */
        0,              /* fx_cs */
        0,              /* __fx_ign0 */
        0,              /* fx_dp */
        0,              /* fx_ds */
        0,              /* __fx_ign1 */
#endif /* __amd64 */
        SSE_MXCSR_INIT  /* fx_mxcsr */
        /* rest of structure is zero */
};

/*CSTYLED*/
#pragma align 64 (avx_initial)

/*
 * Initial kfpu state for AVX used by fpinit()
 */
const struct xsave_state avx_initial = {
        /*
         * The definition below needs to be identical with sse_initial
         * defined above.
         */
        {
                FPU_CW_INIT,    /* fx_fcw */
                0,              /* fx_fsw */
                0,              /* fx_fctw */
                0,              /* fx_fop */
#if defined(__amd64)
                0,              /* fx_rip */
                0,              /* fx_rdp */
#else
                0,              /* fx_eip */
                0,              /* fx_cs */
                0,              /* __fx_ign0 */
                0,              /* fx_dp */
                0,              /* fx_ds */
                0,              /* __fx_ign1 */
#endif /* __amd64 */
                SSE_MXCSR_INIT  /* fx_mxcsr */
                /* rest of structure is zero */
        },
        /*
         * bit0 = 1 for XSTATE_BV to indicate that legacy fields are valid,
         * and CPU should initialize XMM/YMM.
         */
        1,
        {0, 0}  /* These 2 bytes must be zero */
        /* rest of structure is zero */
};

/*
 * mxcsr_mask value (possibly reset in fpu_probe); used to avoid
 * the #gp exception caused by setting unsupported bits in the
 * MXCSR register
 */
uint32_t sse_mxcsr_mask = SSE_MXCSR_MASK_DEFAULT;

/*
 * Initial kfpu state for x87 used by fpinit()
 */
const struct fnsave_state x87_initial = {
        FPU_CW_INIT,    /* f_fcw */
        0,              /* __f_ign0 */
        0,              /* f_fsw */
        0,              /* __f_ign1 */
        0xffff,         /* f_ftw */
        /* rest of structure is zero */
};

#if defined(__amd64)
/*
 * This vector is patched to xsave_ctxt() if we discover we have an
 * XSAVE-capable chip in fpu_probe.
 */
void (*fpsave_ctxt)(void *) = fpxsave_ctxt;
#elif defined(__i386)
/*
 * This vector is patched to fpxsave_ctxt() if we discover we have an
 * SSE-capable chip in fpu_probe(). It is patched to xsave_ctxt
 * if we discover we have an XSAVE-capable chip in fpu_probe.
 */
void (*fpsave_ctxt)(void *) = fpnsave_ctxt;
#endif

static int fpe_sicode(uint_t);
static int fpe_simd_sicode(uint_t);

/*
 * Copy the state of parent lwp's floating point context into the new lwp.
 * Invoked for both fork() and lwp_create().
 *
 * Note that we inherit -only- the control state (e.g. exception masks,
 * rounding, precision control, etc.); the FPU registers are otherwise
 * reset to their initial state.
 */
static void
fp_new_lwp(kthread_id_t t, kthread_id_t ct)
{
        struct fpu_ctx *fp;             /* parent fpu context */
        struct fpu_ctx *cfp;            /* new fpu context */
        struct fxsave_state *fx, *cfx;
#if defined(__i386)
        struct fnsave_state *fn, *cfn;
#endif
        struct xsave_state *cxs;

        ASSERT(fp_kind != FP_NO);

        fp = &t->t_lwp->lwp_pcb.pcb_fpu;
        cfp = &ct->t_lwp->lwp_pcb.pcb_fpu;

        /*
         * If the parent FPU state is still in the FPU hw then save it;
         * conveniently, fp_save() already does this for us nicely.
         */
        fp_save(fp);

        cfp->fpu_flags = FPU_EN | FPU_VALID;
        cfp->fpu_regs.kfpu_status = 0;
        cfp->fpu_regs.kfpu_xstatus = 0;

        switch (fp_save_mech) {
#if defined(__i386)
        case FP_FNSAVE:
                fn = &fp->fpu_regs.kfpu_u.kfpu_fn;
                cfn = &cfp->fpu_regs.kfpu_u.kfpu_fn;
                bcopy(&x87_initial, cfn, sizeof (*cfn));
                cfn->f_fcw = fn->f_fcw;
                break;
#endif
        case FP_FXSAVE:
                fx = &fp->fpu_regs.kfpu_u.kfpu_fx;
                cfx = &cfp->fpu_regs.kfpu_u.kfpu_fx;
                bcopy(&sse_initial, cfx, sizeof (*cfx));
                cfx->fx_mxcsr = fx->fx_mxcsr & ~SSE_MXCSR_EFLAGS;
                cfx->fx_fcw = fx->fx_fcw;
                break;

        case FP_XSAVE:
                cfp->fpu_xsave_mask = fp->fpu_xsave_mask;

                fx = &fp->fpu_regs.kfpu_u.kfpu_xs.xs_fxsave;
                cxs = &cfp->fpu_regs.kfpu_u.kfpu_xs;
                cfx = &cxs->xs_fxsave;

                bcopy(&avx_initial, cxs, sizeof (*cxs));
                cfx->fx_mxcsr = fx->fx_mxcsr & ~SSE_MXCSR_EFLAGS;
                cfx->fx_fcw = fx->fx_fcw;
                cxs->xs_xstate_bv |= (get_xcr(XFEATURE_ENABLED_MASK) &
                    XFEATURE_FP_ALL);
                break;
        default:
                panic("Invalid fp_save_mech");
                /*NOTREACHED*/
        }

        installctx(ct, cfp,
            fpsave_ctxt, NULL, fp_new_lwp, fp_new_lwp, NULL, fp_free);
        /*
         * Now, when the new lwp starts running, it will take a trap
         * that will be handled inline in the trap table to cause
         * the appropriate f*rstor instruction to load the save area we
         * constructed above directly into the hardware.
         */
}

/*
 * Free any state associated with floating point context.
 * Fp_free can be called in three cases:
 * 1) from reaper -> thread_free -> ctxfree -> fp_free
 *      fp context belongs to a thread on deathrow
 *      nothing to do,  thread will never be resumed
 *      thread calling ctxfree is reaper
 *
 * 2) from exec -> ctxfree -> fp_free
 *      fp context belongs to the current thread
 *      must disable fpu, thread calling ctxfree is curthread
 *
 * 3) from restorecontext -> setfpregs -> fp_free
 *      we have a modified context in the memory (lwp->pcb_fpu)
 *      disable fpu and release the fp context for the CPU
 *
 */
/*ARGSUSED*/
void
fp_free(struct fpu_ctx *fp, int isexec)
{
        ASSERT(fp_kind != FP_NO);

        if (fp->fpu_flags & FPU_VALID)
                return;

        kpreempt_disable();
        /*
         * We want to do fpsave rather than fpdisable so that we can
         * keep the fpu_flags as FPU_VALID tracking the CR0_TS bit
         */
        fp->fpu_flags |= FPU_VALID;
        /* If for current thread disable FP to track FPU_VALID */
        if (curthread->t_lwp && fp == &curthread->t_lwp->lwp_pcb.pcb_fpu) {
                /* Clear errors if any to prevent frstor from complaining */
                (void) fperr_reset();
                if (fp_kind & __FP_SSE)
                        (void) fpxerr_reset();
                fpdisable();
        }
        kpreempt_enable();
}

/*
 * Store the floating point state and disable the floating point unit.
 */
void
fp_save(struct fpu_ctx *fp)
{
        ASSERT(fp_kind != FP_NO);

        kpreempt_disable();
        if (!fp || fp->fpu_flags & FPU_VALID) {
                kpreempt_enable();
                return;
        }
        ASSERT(curthread->t_lwp && fp == &curthread->t_lwp->lwp_pcb.pcb_fpu);

        switch (fp_save_mech) {
#if defined(__i386)
        case FP_FNSAVE:
                fpsave(&fp->fpu_regs.kfpu_u.kfpu_fn);
                break;
#endif
        case FP_FXSAVE:
                fpxsave(&fp->fpu_regs.kfpu_u.kfpu_fx);
                break;

        case FP_XSAVE:
                xsave(&fp->fpu_regs.kfpu_u.kfpu_xs, fp->fpu_xsave_mask);
                break;
        default:
                panic("Invalid fp_save_mech");
                /*NOTREACHED*/
        }

        fp->fpu_flags |= FPU_VALID;
        kpreempt_enable();
}

/*
 * Restore the FPU context for the thread:
 * The possibilities are:
 *      1. No active FPU context: Load the new context into the FPU hw
 *         and enable the FPU.
 */
void
fp_restore(struct fpu_ctx *fp)
{
        switch (fp_save_mech) {
#if defined(__i386)
        case FP_FNSAVE:
                fprestore(&fp->fpu_regs.kfpu_u.kfpu_fn);
                break;
#endif
        case FP_FXSAVE:
                fpxrestore(&fp->fpu_regs.kfpu_u.kfpu_fx);
                break;

        case FP_XSAVE:
                xrestore(&fp->fpu_regs.kfpu_u.kfpu_xs, fp->fpu_xsave_mask);
                break;
        default:
                panic("Invalid fp_save_mech");
                /*NOTREACHED*/
        }

        fp->fpu_flags &= ~FPU_VALID;
}


/*
 * Seeds the initial state for the current thread.  The possibilities are:
 *      1. Another process has modified the FPU state before we have done any
 *         initialization: Load the FPU state from the LWP state.
 *      2. The FPU state has not been externally modified:  Load a clean state.
 */
static void
fp_seed(void)
{
        struct fpu_ctx *fp = &ttolwp(curthread)->lwp_pcb.pcb_fpu;

        ASSERT(curthread->t_preempt >= 1);
        ASSERT((fp->fpu_flags & FPU_EN) == 0);

        /*
         * Always initialize a new context and initialize the hardware.
         */
        if (fp_save_mech == FP_XSAVE) {
                fp->fpu_xsave_mask = get_xcr(XFEATURE_ENABLED_MASK) &
                    XFEATURE_FP_ALL;
        }

        installctx(curthread, fp,
            fpsave_ctxt, NULL, fp_new_lwp, fp_new_lwp, NULL, fp_free);
        fpinit();

        /*
         * If FPU_VALID is set, it means someone has modified registers via
         * /proc.  In this case, restore the current lwp's state.
         */
        if (fp->fpu_flags & FPU_VALID)
                fp_restore(fp);

        ASSERT((fp->fpu_flags & FPU_VALID) == 0);
        fp->fpu_flags = FPU_EN;
}

/*
 * This routine is called from trap() when User thread takes No Extension
 * Fault. The possiblities are:
 *      1. User thread has executed a FP instruction for the first time.
 *         Save current FPU context if any. Initialize FPU, setup FPU
 *         context for the thread and enable FP hw.
 *      2. Thread's pcb has a valid FPU state: Restore the FPU state and
 *         enable FP hw.
 *
 * Note that case #2 is inlined in the trap table.
 */
int
fpnoextflt(struct regs *rp)
{
        struct fpu_ctx *fp = &ttolwp(curthread)->lwp_pcb.pcb_fpu;

#if !defined(__lint)
        ASSERT(sizeof (struct fxsave_state) == 512 &&
            sizeof (struct fnsave_state) == 108);
        ASSERT((offsetof(struct fxsave_state, fx_xmm[0]) & 0xf) == 0);

        ASSERT(sizeof (struct xsave_state) >= AVX_XSAVE_SIZE);

#if defined(__i386)
        ASSERT(sizeof (struct fpu) == sizeof (struct __old_fpu));
#endif  /* __i386 */
#endif  /* !__lint */

        /*
         * save area MUST be 16-byte aligned, else will page fault
         */
        ASSERT(((uintptr_t)(&fp->fpu_regs.kfpu_u.kfpu_fx) & 0xf) == 0);

        kpreempt_disable();
        /*
         * Now we can enable the interrupts.
         * (NOTE: fp-no-coprocessor comes thru interrupt gate)
         */
        sti();

        if (!fpu_exists) { /* check for FPU hw exists */
                if (fp_kind == FP_NO) {
                        uint32_t inst;

                        /*
                         * When the system has no floating point support,
                         * i.e. no FP hardware and no emulator, skip the
                         * two kinds of FP instruction that occur in
                         * fpstart.  Allows processes that do no real FP
                         * to run normally.
                         */
                        if (fuword32((void *)rp->r_pc, &inst) != -1 &&
                            ((inst & 0xFFFF) == 0x7dd9 ||
                            (inst & 0xFFFF) == 0x6dd9)) {
                                rp->r_pc += 3;
                                kpreempt_enable();
                                return (0);
                        }
                }

                /*
                 * If we have neither a processor extension nor
                 * an emulator, kill the process OR panic the kernel.
                 */
                kpreempt_enable();
                return (1); /* error */
        }

#if !defined(__xpv)     /* XXPV Is this ifdef needed now? */
        /*
         * A paranoid cross-check: for the SSE case, ensure that %cr4 is
         * configured to enable fully fledged (%xmm) fxsave/fxrestor on
         * this CPU.  For the non-SSE case, ensure that it isn't.
         */
        ASSERT(((fp_kind & __FP_SSE) &&
            (getcr4() & CR4_OSFXSR) == CR4_OSFXSR) ||
            (!(fp_kind & __FP_SSE) &&
            (getcr4() & (CR4_OSXMMEXCPT|CR4_OSFXSR)) == 0));
#endif

        if (fp->fpu_flags & FPU_EN) {
                /* case 2 */
                fp_restore(fp);
        } else {
                /* case 1 */
                fp_seed();
        }
        kpreempt_enable();
        return (0);
}


/*
 * Handle a processor extension overrun fault
 * Returns non zero for error.
 *
 * XXX  Shouldn't this just be abolished given that we're not supporting
 *      anything prior to Pentium?
 */

/* ARGSUSED */
int
fpextovrflt(struct regs *rp)
{
#if !defined(__xpv)             /* XXPV Do we need this ifdef either */
        ulong_t cur_cr0;

        ASSERT(fp_kind != FP_NO);

        cur_cr0 = getcr0();
        fpinit();               /* initialize the FPU hardware */
        setcr0(cur_cr0);
#endif
        sti();
        return (1);             /* error, send SIGSEGV signal to the thread */
}

/*
 * Handle a processor extension error fault
 * Returns non zero for error.
 */

/*ARGSUSED*/
int
fpexterrflt(struct regs *rp)
{
        uint32_t fpcw, fpsw;
        fpu_ctx_t *fp = &ttolwp(curthread)->lwp_pcb.pcb_fpu;

        ASSERT(fp_kind != FP_NO);

        /*
         * Now we can enable the interrupts.
         * (NOTE: x87 fp exceptions come thru interrupt gate)
         */
        sti();

        if (!fpu_exists)
                return (FPE_FLTINV);

        /*
         * Do an unconditional save of the FP state.  If it's dirty (TS=0),
         * it'll be saved into the fpu context area passed in (that of the
         * current thread).  If it's not dirty (it may not be, due to
         * an intervening save due to a context switch between the sti(),
         * above and here, then it's safe to just use the stored values in
         * the context save area to determine the cause of the fault.
         */
        fp_save(fp);

        /* clear exception flags in saved state, as if by fnclex */
        switch (fp_save_mech) {
#if defined(__i386)
        case FP_FNSAVE:
                fpsw = fp->fpu_regs.kfpu_u.kfpu_fn.f_fsw;
                fpcw = fp->fpu_regs.kfpu_u.kfpu_fn.f_fcw;
                fp->fpu_regs.kfpu_u.kfpu_fn.f_fsw &= ~FPS_SW_EFLAGS;
                break;
#endif

        case FP_FXSAVE:
                fpsw = fp->fpu_regs.kfpu_u.kfpu_fx.fx_fsw;
                fpcw = fp->fpu_regs.kfpu_u.kfpu_fx.fx_fcw;
                fp->fpu_regs.kfpu_u.kfpu_fx.fx_fsw &= ~FPS_SW_EFLAGS;
                break;

        case FP_XSAVE:
                fpsw = fp->fpu_regs.kfpu_u.kfpu_xs.xs_fxsave.fx_fsw;
                fpcw = fp->fpu_regs.kfpu_u.kfpu_xs.xs_fxsave.fx_fcw;
                fp->fpu_regs.kfpu_u.kfpu_xs.xs_fxsave.fx_fsw &= ~FPS_SW_EFLAGS;
                /*
                 * Always set LEGACY_FP as it may have been cleared by XSAVE
                 * instruction
                 */
                fp->fpu_regs.kfpu_u.kfpu_xs.xs_xstate_bv |= XFEATURE_LEGACY_FP;
                break;
        default:
                panic("Invalid fp_save_mech");
                /*NOTREACHED*/
        }

        fp->fpu_regs.kfpu_status = fpsw;

        if ((fpsw & FPS_ES) == 0)
                return (0);             /* No exception */

        /*
         * "and" the exception flags with the complement of the mask
         * bits to determine which exception occurred
         */
        return (fpe_sicode(fpsw & ~fpcw & 0x3f));
}

/*
 * Handle an SSE/SSE2 precise exception.
 * Returns a non-zero sicode for error.
 */
/*ARGSUSED*/
int
fpsimderrflt(struct regs *rp)
{
        uint32_t mxcsr, xmask;
        fpu_ctx_t *fp = &ttolwp(curthread)->lwp_pcb.pcb_fpu;

        ASSERT(fp_kind & __FP_SSE);

        /*
         * NOTE: Interrupts are disabled during execution of this
         * function.  They are enabled by the caller in trap.c.
         */

        /*
         * The only way we could have gotten here if there is no FP unit
         * is via a user executing an INT $19 instruction, so there is
         * no fault in that case.
         */
        if (!fpu_exists)
                return (0);

        /*
         * Do an unconditional save of the FP state.  If it's dirty (TS=0),
         * it'll be saved into the fpu context area passed in (that of the
         * current thread).  If it's not dirty, then it's safe to just use
         * the stored values in the context save area to determine the
         * cause of the fault.
         */
        fp_save(fp);            /* save the FPU state */

        mxcsr = fp->fpu_regs.kfpu_u.kfpu_fx.fx_mxcsr;

        fp->fpu_regs.kfpu_status = fp->fpu_regs.kfpu_u.kfpu_fx.fx_fsw;

        fp->fpu_regs.kfpu_xstatus = mxcsr;

        /*
         * compute the mask that determines which conditions can cause
         * a #xm exception, and use this to clean the status bits so that
         * we can identify the true cause of this one.
         */
        xmask = (mxcsr >> 7) & SSE_MXCSR_EFLAGS;
        return (fpe_simd_sicode((mxcsr & SSE_MXCSR_EFLAGS) & ~xmask));
}

/*
 * In the unlikely event that someone is relying on this subcode being
 * FPE_FLTILL for denormalize exceptions, it can always be patched back
 * again to restore old behaviour.
 */
int fpe_fltden = FPE_FLTDEN;

/*
 * Map from the FPU status word to the FP exception si_code.
 */
static int
fpe_sicode(uint_t sw)
{
        if (sw & FPS_IE)
                return (FPE_FLTINV);
        if (sw & FPS_ZE)
                return (FPE_FLTDIV);
        if (sw & FPS_DE)
                return (fpe_fltden);
        if (sw & FPS_OE)
                return (FPE_FLTOVF);
        if (sw & FPS_UE)
                return (FPE_FLTUND);
        if (sw & FPS_PE)
                return (FPE_FLTRES);
        return (FPE_FLTINV);    /* default si_code for other exceptions */
}

/*
 * Map from the SSE status word to the FP exception si_code.
 */
static int
fpe_simd_sicode(uint_t sw)
{
        if (sw & SSE_IE)
                return (FPE_FLTINV);
        if (sw & SSE_ZE)
                return (FPE_FLTDIV);
        if (sw & SSE_DE)
                return (FPE_FLTDEN);
        if (sw & SSE_OE)
                return (FPE_FLTOVF);
        if (sw & SSE_UE)
                return (FPE_FLTUND);
        if (sw & SSE_PE)
                return (FPE_FLTRES);
        return (FPE_FLTINV);    /* default si_code for other exceptions */
}

/*
 * This routine is invoked as part of libc's __fpstart implementation
 * via sysi86(2).
 *
 * It may be called -before- any context has been assigned in which case
 * we try and avoid touching the hardware.  Or it may be invoked well
 * after the context has been assigned and fiddled with, in which case
 * just tweak it directly.
 */
void
fpsetcw(uint16_t fcw, uint32_t mxcsr)
{
        struct fpu_ctx *fp = &curthread->t_lwp->lwp_pcb.pcb_fpu;
        struct fxsave_state *fx;

        if (!fpu_exists || fp_kind == FP_NO)
                return;

        if ((fp->fpu_flags & FPU_EN) == 0) {
                if (fcw == FPU_CW_INIT && mxcsr == SSE_MXCSR_INIT) {
                        /*
                         * Common case.  Floating point unit not yet
                         * enabled, and kernel already intends to initialize
                         * the hardware the way the caller wants.
                         */
                        return;
                }
                /*
                 * Hmm.  Userland wants a different default.
                 * Do a fake "first trap" to establish the context, then
                 * handle as if we already had a context before we came in.
                 */
                kpreempt_disable();
                fp_seed();
                kpreempt_enable();
        }

        /*
         * Ensure that the current hardware state is flushed back to the
         * pcb, then modify that copy.  Next use of the fp will
         * restore the context.
         */
        fp_save(fp);

        switch (fp_save_mech) {
#if defined(__i386)
        case FP_FNSAVE:
                fp->fpu_regs.kfpu_u.kfpu_fn.f_fcw = fcw;
                break;
#endif
        case FP_FXSAVE:
                fx = &fp->fpu_regs.kfpu_u.kfpu_fx;
                fx->fx_fcw = fcw;
                fx->fx_mxcsr = sse_mxcsr_mask & mxcsr;
                break;

        case FP_XSAVE:
                fx = &fp->fpu_regs.kfpu_u.kfpu_xs.xs_fxsave;
                fx->fx_fcw = fcw;
                fx->fx_mxcsr = sse_mxcsr_mask & mxcsr;
                /*
                 * Always set LEGACY_FP as it may have been cleared by XSAVE
                 * instruction
                 */
                fp->fpu_regs.kfpu_u.kfpu_xs.xs_xstate_bv |= XFEATURE_LEGACY_FP;
                break;
        default:
                panic("Invalid fp_save_mech");
                /*NOTREACHED*/
        }
}