Re: [PATCH v1 1/1] target/i386: Mask xstate_bv based on the cpu enabled features

[Leonardo Bras Soares Passos <leobras@redhat.com>]

Hello David Edmondson and Igor Memmedov,

Thank you for the feedback!

For some reason I did not get your comments in my email.
I could only notice them when I opened Patchwork to get the link.

Sorry for the delay. I will do my best to address them in a few minutes.

Best regards,
Leo

On Sat, Jan 29, 2022 at 6:47 AM Leonardo Bras <leobras@redhat.com> wrote:
>
> The following steps describe a migration bug:
> 1 - Bring up a VM with -cpu EPYC on a host with EPYC-Milan cpu
> 2 - Migrate to a host with EPYC-Naples cpu
>
> The guest kernel crashes shortly after the migration.
>
> The crash happens due to a fault caused by XRSTOR:
> A set bit in XSTATE_BV is not set in XCR0.
> The faulting bit is FEATURE_PKRU (enabled in Milan, but not in Naples)
>
> To avoid this kind of bug:
> In kvm_get_xsave, mask-out from xstate_bv any bits that are not set in
> current vcpu's features.
>
> This keeps cpu->env->xstate_bv with feature bits compatible with any
> host machine capable of running the vcpu model.
>
> Signed-off-by: Leonardo Bras <leobras@redhat.com>
> ---
>  target/i386/xsave_helper.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/target/i386/xsave_helper.c b/target/i386/xsave_helper.c
> index ac61a96344..0628226234 100644
> --- a/target/i386/xsave_helper.c
> +++ b/target/i386/xsave_helper.c
> @@ -167,7 +167,7 @@ void x86_cpu_xrstor_all_areas(X86CPU *cpu, const void *buf, uint32_t buflen)
>          env->xmm_regs[i].ZMM_Q(1) = ldq_p(xmm + 8);
>      }
>
> -    env->xstate_bv = header->xstate_bv;
> +    env->xstate_bv = header->xstate_bv & env->features[FEAT_XSAVE_COMP_LO];
>
>      e = &x86_ext_save_areas[XSTATE_YMM_BIT];
>      if (e->size && e->offset) {
> --
> 2.34.1
>