1 #include <linux/linkage.h>
2 #include <linux/lguest.h>
3 #include <asm/lguest_hcall.h>
4 #include <asm/asm-offsets.h>
5 #include <asm/thread_info.h>
6 #include <asm/processor-flags.h>
9 * Our story starts with the kernel booting into startup_32 in
10 * arch/x86/kernel/head_32.S. It expects a boot header, which is created by
11 * the bootloader (the Launcher in our case).
13 * The startup_32 function does very little: it clears the uninitialized global
14 * C variables which we expect to be zero (ie. BSS) and then copies the boot
15 * header and kernel command line somewhere safe. Finally it checks the
16 * 'hardware_subarch' field. This was introduced in 2.6.24 for lguest and Xen:
17 * if it's set to '1' (lguest's assigned number), then it calls us here.
19 * WARNING: be very careful here! We're running at addresses equal to physical
20 * addesses (around 0), not above PAGE_OFFSET as most code expectes
21 * (eg. 0xC0000000). Jumps are relative, so they're OK, but we can't touch any
22 * data without remembering to subtract __PAGE_OFFSET!
24 * The .section line puts this code in .init.text so it will be discarded after
27 .section .init.text, "ax", @progbits
30 * We make the "initialization" hypercall now to tell the Host about
31 * us, and also find out where it put our page tables.
33 movl $LHCALL_LGUEST_INIT, %eax
34 movl $lguest_data - __PAGE_OFFSET, %ebx
35 .byte 0x0f,0x01,0xc1 /* KVM_HYPERCALL */
37 /* Set up the initial stack so we can run C code. */
38 movl $(init_thread_union+THREAD_SIZE),%esp
40 /* Jumps are relative: we're running __PAGE_OFFSET too low. */
41 jmp lguest_init+__PAGE_OFFSET
44 * We create a macro which puts the assembler code between lgstart_ and lgend_
45 * markers. These templates are put in the .text section: they can't be
46 * discarded after boot as we may need to patch modules, too.
49 #define LGUEST_PATCH(name, insns...) \
50 lgstart_##name: insns; lgend_##name:; \
51 .globl lgstart_##name; .globl lgend_##name
53 LGUEST_PATCH(cli, movl $0, lguest_data+LGUEST_DATA_irq_enabled)
54 LGUEST_PATCH(pushf, movl lguest_data+LGUEST_DATA_irq_enabled, %eax)
57 * But using those wrappers is inefficient (we'll see why that doesn't matter
58 * for save_fl and irq_disable later). If we write our routines carefully in
59 * assembler, we can avoid clobbering any registers and avoid jumping through
60 * the wrapper functions.
62 * I skipped over our first piece of assembler, but this one is worth studying
63 * in a bit more detail so I'll describe in easy stages. First, the routine to
68 * The reverse of irq_disable, this sets lguest_data.irq_enabled to
69 * X86_EFLAGS_IF (ie. "Interrupts enabled").
71 movl $X86_EFLAGS_IF, lguest_data+LGUEST_DATA_irq_enabled
73 * But now we need to check if the Host wants to know: there might have
74 * been interrupts waiting to be delivered, in which case it will have
75 * set lguest_data.irq_pending to X86_EFLAGS_IF. If it's not zero, we
76 * jump to send_interrupts, otherwise we're done.
78 testl $0, lguest_data+LGUEST_DATA_irq_pending
81 * One cool thing about x86 is that you can do many things without using
82 * a register. In this case, the normal path hasn't needed to save or
83 * restore any registers at all!
88 * OK, now we need a register: eax is used for the hypercall number,
89 * which is LHCALL_SEND_INTERRUPTS.
91 * We used not to bother with this pending detection at all, which was
92 * much simpler. Sooner or later the Host would realize it had to
93 * send us an interrupt. But that turns out to make performance 7
94 * times worse on a simple tcp benchmark. So now we do this the hard
98 movl $LHCALL_SEND_INTERRUPTS, %eax
100 * This is a vmcall instruction (same thing that KVM uses). Older
101 * assembler versions might not know the "vmcall" instruction, so we
102 * create one manually here.
104 .byte 0x0f,0x01,0xc1 /* KVM_HYPERCALL */
109 * Finally, the "popf" or "restore flags" routine. The %eax register holds the
110 * flags (in practice, either X86_EFLAGS_IF or 0): if it's X86_EFLAGS_IF we're
111 * enabling interrupts again, if it's 0 we're leaving them off.
114 /* This is just "lguest_data.irq_enabled = flags;" */
115 movl %eax, lguest_data+LGUEST_DATA_irq_enabled
117 * Now, if the %eax value has enabled interrupts and
118 * lguest_data.irq_pending is set, we want to tell the Host so it can
119 * deliver any outstanding interrupts. Fortunately, both values will
120 * be X86_EFLAGS_IF (ie. 512) in that case, and the "testl"
121 * instruction will AND them together for us. If both are set, we
122 * jump to send_interrupts.
124 testl lguest_data+LGUEST_DATA_irq_pending, %eax
126 /* Again, the normal path has used no extra registers. Clever, huh? */
129 /* These demark the EIP range where host should never deliver interrupts. */
130 .global lguest_noirq_start
131 .global lguest_noirq_end
134 * When the Host reflects a trap or injects an interrupt into the Guest, it
135 * sets the eflags interrupt bit on the stack based on lguest_data.irq_enabled,
136 * so the Guest iret logic does the right thing when restoring it. However,
137 * when the Host sets the Guest up for direct traps, such as system calls, the
138 * processor is the one to push eflags onto the stack, and the interrupt bit
139 * will be 1 (in reality, interrupts are always enabled in the Guest).
141 * This turns out to be harmless: the only trap which should happen under Linux
142 * with interrupts disabled is Page Fault (due to our lazy mapping of vmalloc
143 * regions), which has to be reflected through the Host anyway. If another
144 * trap *does* go off when interrupts are disabled, the Guest will panic, and
145 * we'll never get to this iret!
149 * There is one final paravirt_op that the Guest implements, and glancing at it
150 * you can see why I left it to last. It's *cool*! It's in *assembler*!
152 * The "iret" instruction is used to return from an interrupt or trap. The
153 * stack looks like this:
155 * old code segment & privilege level
156 * old processor flags ("eflags")
158 * The "iret" instruction pops those values off the stack and restores them all
159 * at once. The only problem is that eflags includes the Interrupt Flag which
160 * the Guest can't change: the CPU will simply ignore it when we do an "iret".
161 * So we have to copy eflags from the stack to lguest_data.irq_enabled before
164 * There are two problems with this: firstly, we need to use a register to do
165 * the copy and secondly, the whole thing needs to be atomic. The first
166 * problem is easy to solve: push %eax on the stack so we can use it, and then
167 * restore it at the end just before the real "iret".
169 * The second is harder: copying eflags to lguest_data.irq_enabled will turn
170 * interrupts on before we're finished, so we could be interrupted before we
171 * return to userspace or wherever. Our solution to this is to surround the
172 * code with lguest_noirq_start: and lguest_noirq_end: labels. We tell the
173 * Host that it is *never* to interrupt us there, even if interrupts seem to be
181 * Note the %ss: segment prefix here. Normal data accesses use the
182 * "ds" segment, but that will have already been restored for whatever
183 * we're returning to (such as userspace): we can't trust it. The %ss:
184 * prefix makes sure we use the stack segment, which is still valid.
186 movl %eax,%ss:lguest_data+LGUEST_DATA_irq_enabled