linux-kernel - Re: [PATCH] KVM: x86: Always set kvm_run->if

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Message-ID: <CAA03e5E7-ns7w9B9Tu7pSWzCo0Nh7Ba5jwQXcn_XYPf_reRq9Q@mail.gmail.com>
Date:   Tue, 7 Dec 2021 07:14:52 -0800
From:   Marc Orr <marcorr@...gle.com>
To:     Tom Lendacky <Thomas.Lendacky@....com>
Cc:     pbonzini@...hat.com, seanjc@...gle.com, vkuznets@...hat.com,
        wanpengli@...cent.com, jmattson@...gle.com, joro@...tes.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: Always set kvm_run->if_flag

On Tue, Dec 7, 2021 at 6:43 AM Tom Lendacky <thomas.lendacky@....com> wrote:
>
> On 12/6/21 10:31 PM, Marc Orr wrote:
> > The kvm_run struct's if_flag is apart of the userspace/kernel API. The
> > SEV-ES patches failed to set this flag because it's no longer needed by
> > QEMU (according to the comment in the source code). However, other
> > hypervisors may make use of this flag. Therefore, set the flag for
> > guests with encrypted regiesters (i.e., with guest_state_protected set).
> >
> > Fixes: f1c6366e3043 ("KVM: SVM: Add required changes to support intercepts under SEV-ES")
> > Signed-off-by: Marc Orr <marcorr@...gle.com>
> > ---
> >   arch/x86/include/asm/kvm-x86-ops.h | 1 +
> >   arch/x86/include/asm/kvm_host.h    | 1 +
> >   arch/x86/kvm/svm/svm.c             | 8 ++++++++
> >   arch/x86/kvm/vmx/vmx.c             | 6 ++++++
> >   arch/x86/kvm/x86.c                 | 9 +--------
> >   5 files changed, 17 insertions(+), 8 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/kvm-x86-ops.h b/arch/x86/include/asm/kvm-x86-ops.h
> > index cefe1d81e2e8..9e50da3ed01a 100644
> > --- a/arch/x86/include/asm/kvm-x86-ops.h
> > +++ b/arch/x86/include/asm/kvm-x86-ops.h
> > @@ -47,6 +47,7 @@ KVM_X86_OP(set_dr7)
> >   KVM_X86_OP(cache_reg)
> >   KVM_X86_OP(get_rflags)
> >   KVM_X86_OP(set_rflags)
> > +KVM_X86_OP(get_if_flag)
> >   KVM_X86_OP(tlb_flush_all)
> >   KVM_X86_OP(tlb_flush_current)
> >   KVM_X86_OP_NULL(tlb_remote_flush)
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index 860ed500580c..a7f868ff23e7 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -1349,6 +1349,7 @@ struct kvm_x86_ops {
> >       void (*cache_reg)(struct kvm_vcpu *vcpu, enum kvm_reg reg);
> >       unsigned long (*get_rflags)(struct kvm_vcpu *vcpu);
> >       void (*set_rflags)(struct kvm_vcpu *vcpu, unsigned long rflags);
> > +     bool (*get_if_flag)(struct kvm_vcpu *vcpu);
> >
> >       void (*tlb_flush_all)(struct kvm_vcpu *vcpu);
> >       void (*tlb_flush_current)(struct kvm_vcpu *vcpu);
> > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > index d0f68d11ec70..91608f8c0cde 100644
> > --- a/arch/x86/kvm/svm/svm.c
> > +++ b/arch/x86/kvm/svm/svm.c
> > @@ -1585,6 +1585,13 @@ static void svm_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags)
> >       to_svm(vcpu)->vmcb->save.rflags = rflags;
> >   }
> >
> > +static bool svm_get_if_flag(struct kvm_vcpu *vcpu)
> > +{
> > +     struct vmcb *vmcb = to_svm(vcpu)->vmcb;
> > +
> > +     return !!(vmcb->control.int_state & SVM_GUEST_INTERRUPT_MASK);
>
> I'm not sure if this is always valid to use for non SEV-ES guests. Maybe
> the better thing would be:
>
>         return sev_es_guest(vcpu->kvm) ? vmcb->control.int_state & SVM_GUEST_INTERRUPT_MASK
>                                        : kvm_get_rflags(vcpu) & X86_EFLAGS_IF;
>
> (Since this function returns a bool, I don't think you need the !!)

I had the same reservations when writing the patch. (Why fix what's
not broken.) The reason I wrote the patch this way is based on what I
read in APM vol2: Appendix B Layout of VMCB: "GUEST_INTERRUPT_MASK -
Value of the RFLAGS.IF bit for the guest."

Also, I had _thought_ that `svm_interrupt_allowed()` -- the
AMD-specific function used to populate `ready_for_interrupt_injection`
-- was relying on `GUEST_INTERRUPT_MASK`. But now I'm reading the code
again, and I realized I was overly focused on the SEV-ES handling.
That code is actually extracting the IF bit from the RFLAGS register
in the same way you've proposed here.

Changing the patch as you've suggested SGTM. I can send out a v2. I'll
wait a day or two to see if there are any other comments first. I
guess the alternative would be to change `svm_interrupt_blocked()` to
solely use the `SVM_GUEST_INTERRUPT_MASK`. If we were confident that
it was sufficient, it would be a nice little cleanup. But regardless,
I think we should keep the code introduced by this patch consistent
with `svm_interrupt_blocked()`.

Also, noted on the `!!` not being needed when returning from a bool
function. I'll keep this in mind in the future. Thanks!