Commit 15d68c5e authored by Greg Kurz's avatar Greg Kurz Committed by David Gibson
Browse files

target/ppc: Improve comment of bcctr used for spectre v2 mitigation 



Signed-off-by: default avatarGreg Kurz <groug@kaod.org>
Message-Id: <155359567174.1794128.3183997593369465355.stgit@bahia.lan>
Signed-off-by: default avatarDavid Gibson <david@gibson.dropbear.id.au>
parent d0db7cad

target/ppc/translate.c

+9 −1
Original line number Diff line number Diff line
@@ -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);