| Message ID | 155359567174.1794128.3183997593369465355.stgit@bahia.lan |
|---|---|
| State | New |
| Headers | show |
| Series | target/ppc: Improve comment of bcctr used for spectre v2 mitigation | expand |
On Tue, Mar 26, 2019 at 11:21:11AM +0100, Greg Kurz wrote: > Signed-off-by: Greg Kurz <groug@kaod.org> Applied, thanks. > --- > 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 576210d901ad..badc1ae1a35d 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); >
diff --git a/target/ppc/translate.c b/target/ppc/translate.c index 576210d901ad..badc1ae1a35d 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);
Signed-off-by: Greg Kurz <groug@kaod.org> --- target/ppc/translate.c | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-)