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

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Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <155359567174.1794128.3183997593369465355.stgit@bahia.lan>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
This commit is contained in:
Greg Kurz 2019-03-26 11:21:11 +01:00 committed by David Gibson
parent d0db7caddb
commit 15d68c5e1d
1 changed files with 9 additions and 1 deletions
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10
target/ppc/translate.c
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@ -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);