Re: [Qemu-devel] [PATCH] target/ppc: Improve comment of bcctr used for spectre v2 mitigation

[David Gibson]

On Tue, Mar 26, 2019 at 11:21:11AM +0100, Greg Kurz wrote:
> Signed-off-by: Greg Kurz 

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);
> 

-- 
David Gibson| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson


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[Qemu-devel] [PATCH] target/ppc: Improve comment of bcctr used for spectre v2 mitigation

[Greg Kurz]

Signed-off-by: Greg Kurz 
---
 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);