linux-kernel - Re: [PATCH] KVM: VMX: add MSR_IA32_TSX_CTRL into msrs_to

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Date:   Wed, 3 May 2023 10:01:19 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Mingwei Zhang <mizhang@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: VMX: add MSR_IA32_TSX_CTRL into msrs_to_save

On Wed, May 3, 2023 at 9:25 AM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Wed, May 03, 2023, Mingwei Zhang wrote:
> > Add MSR_IA32_TSX_CTRL into msrs_to_save[] to explicitly tell userspace to
> > save/restore the register value during migration. Missing this may cause
> > userspace that relies on KVM ioctl(KVM_GET_MSR_INDEX_LIST) fail to port the
> > value to the target VM.
> >
> > Fixes: b07a5c53d42a ("KVM: vmx: use MSR_IA32_TSX_CTRL to hard-disable TSX on guest that lack it")
> > Reported-by: Jim Mattson <jmattson@...gle.com>
> > Signed-off-by: Mingwei Zhang <mizhang@...gle.com>
> > ---
> >  arch/x86/kvm/x86.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 237c483b1230..2236cfee4b7a 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -1431,7 +1431,7 @@ static const u32 msrs_to_save_base[] = {
> >  #endif
> >       MSR_IA32_TSC, MSR_IA32_CR_PAT, MSR_VM_HSAVE_PA,
> >       MSR_IA32_FEAT_CTL, MSR_IA32_BNDCFGS, MSR_TSC_AUX,
> > -     MSR_IA32_SPEC_CTRL,
> > +     MSR_IA32_SPEC_CTRL, MSR_IA32_TSX_CTRL,
> >       MSR_IA32_RTIT_CTL, MSR_IA32_RTIT_STATUS, MSR_IA32_RTIT_CR3_MATCH,
> >       MSR_IA32_RTIT_OUTPUT_BASE, MSR_IA32_RTIT_OUTPUT_MASK,
> >       MSR_IA32_RTIT_ADDR0_A, MSR_IA32_RTIT_ADDR0_B,
> > --
>
> Hmm, KVM shouldn't report the MSR if it can't be written by the guest.  Over-

I think you mean to say that KVM shouldn't report the MSR if it can't
be written by *any* guest. KVM_GET_MSR_INDEX_LIST is a device ioctl,
so it isn't capable of filtering out MSRs that can't be written by
*the* guest, for some occurrence of "the."

> reporting won't cause functional issues, and the odds of the MSR existing but not
> being reported in ARCH_CAPILIBITES are basically zilch, but IMO it's worth adding
> the check if only to document when the MSRs is fully supported.
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index e7f78fe79b32..d8608c6753ff 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -7152,6 +7152,10 @@ static void kvm_probe_msr_to_save(u32 msr_index)
>                 if (!kvm_cpu_cap_has(X86_FEATURE_XFD))
>                         return;
>                 break;
> +       case MSR_IA32_TSX_CTRL:
> +               if (!(kvm_get_arch_capabilities() & ARCH_CAP_TSX_CTRL_MSR))
> +                       return;
> +               break;
>         default:
>                 break;
>         }