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

[Marc Orr]

On Tue, Dec 7, 2021 at 8:00 AM Tom Lendacky <thomas.lendacky@xxxxxxx> wrote:
>
> 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.

Got it now. Then the change you suggested is a must! Thanks, Tom.