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 <address@hidden>

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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