Re: [PATCH] KVM: x86: Always set kvm_run->if_flag

[Tom Lendacky]

On 12/7/21 9:14 AM, Marc Orr wrote:
> On Tue, Dec 7, 2021 at 6:43 AM Tom Lendacky <thomas.lendacky@xxxxxxx> 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@xxxxxxxxxx>
> > > ---
> > >    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."

I just verified with the hardware team that this flag is indeed only set 
for a guest with protected state (SEV-ES / SEV-SNP). An update to the APM 
will be made.

Thanks,
Tom

> 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!