From 15d68c5e1d910498d38c219565b56a1b3dace95d Mon Sep 17 00:00:00 2001 From: Greg Kurz Date: Tue, 26 Mar 2019 11:21:11 +0100 Subject: [PATCH] target/ppc: Improve comment of bcctr used for spectre v2 mitigation Signed-off-by: Greg Kurz Message-Id: <155359567174.1794128.3183997593369465355.stgit@bahia.lan> Signed-off-by: David Gibson --- target/ppc/translate.c | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-) diff --git a/target/ppc/translate.c b/target/ppc/translate.c index 576210d901..badc1ae1a3 100644 --- a/target/ppc/translate.c +++ b/target/ppc/translate.c @@ -3753,7 +3753,15 @@ static void gen_bcond(DisasContext *ctx, int type) * All ISAs up to v3 describe this form of bcctr as invalid but * some processors, ie. 64-bit server processors compliant with * arch 2.x, do implement a "test and decrement" logic instead, - * as described in their respective UMs. + * as described in their respective UMs. This logic involves CTR + * to act as both the branch target and a counter, which makes + * it basically useless and thus never used in real code. + * + * This form was hence chosen to trigger extra micro-architectural + * side-effect on real HW needed for the Spectre v2 workaround. + * It is up to guests that implement such workaround, ie. linux, to + * use this form in a way it just triggers the side-effect without + * doing anything else harmful. */ if (unlikely(!is_book3s_arch2x(ctx))) { gen_inval_exception(ctx, POWERPC_EXCP_INVAL_INVAL); -- 2.11.4.GIT