Re: [PATCH v2 2/5] KVM: VMX: Use wrmsr for switching between guest and host IA32_XSS

[Jim Mattson <jmattson@google.com>]

On Mon, Oct 14, 2019 at 12:05 PM Sean Christopherson
<sean.j.christopherson@intel.com> wrote:
>
> On Sat, Oct 12, 2019 at 10:36:15AM -0700, Jim Mattson wrote:
> > On Fri, Oct 11, 2019 at 5:18 PM Sean Christopherson
> > <sean.j.christopherson@intel.com> wrote:
> > >
> > > On Fri, Oct 11, 2019 at 12:40:29PM -0700, Aaron Lewis wrote:
> > > > Set IA32_XSS for the guest and host during VM Enter and VM Exit
> > > > transitions rather than by using the MSR-load areas.
> > > >
> > > > By moving away from using the MSR-load area we can have a unified
> > > > solution for both AMD and Intel.
> > > >
> > > > Reviewed-by: Jim Mattson <jmattson@google.com>
> > > > Signed-off-by: Aaron Lewis <aaronlewis@google.com>
> > > > ---
> > > >  arch/x86/include/asm/kvm_host.h |  1 +
> > > >  arch/x86/kvm/svm.c              |  7 +++++--
> > > >  arch/x86/kvm/vmx/vmx.c          | 22 ++++++++++------------
> > > >  arch/x86/kvm/x86.c              | 23 +++++++++++++++++++----
> > > >  arch/x86/kvm/x86.h              |  4 ++--
> > > >  5 files changed, 37 insertions(+), 20 deletions(-)
> > > >
> > > > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > > > index 50eb430b0ad8..634c2598e389 100644
> > > > --- a/arch/x86/include/asm/kvm_host.h
> > > > +++ b/arch/x86/include/asm/kvm_host.h
> > > > @@ -562,6 +562,7 @@ struct kvm_vcpu_arch {
> > > >       u64 smbase;
> > > >       u64 smi_count;
> > > >       bool tpr_access_reporting;
> > > > +     bool xsaves_enabled;
> > > >       u64 ia32_xss;
> > > >       u64 microcode_version;
> > > >       u64 arch_capabilities;
> > > > diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> > > > index f8ecb6df5106..da69e95beb4d 100644
> > > > --- a/arch/x86/kvm/svm.c
> > > > +++ b/arch/x86/kvm/svm.c
> > > > @@ -5628,7 +5628,7 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
> > > >       svm->vmcb->save.cr2 = vcpu->arch.cr2;
> > > >
> > > >       clgi();
> > > > -     kvm_load_guest_xcr0(vcpu);
> > > > +     kvm_load_guest_xsave_controls(vcpu);
> > > >
> > > >       if (lapic_in_kernel(vcpu) &&
> > > >               vcpu->arch.apic->lapic_timer.timer_advance_ns)
> > > > @@ -5778,7 +5778,7 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
> > > >       if (unlikely(svm->vmcb->control.exit_code == SVM_EXIT_NMI))
> > > >               kvm_before_interrupt(&svm->vcpu);
> > > >
> > > > -     kvm_put_guest_xcr0(vcpu);
> > > > +     kvm_load_host_xsave_controls(vcpu);
> > > >       stgi();
> > > >
> > > >       /* Any pending NMI will happen here */
> > > > @@ -5887,6 +5887,9 @@ static void svm_cpuid_update(struct kvm_vcpu *vcpu)
> > > >  {
> > > >       struct vcpu_svm *svm = to_svm(vcpu);
> > > >
> > > > +     vcpu->arch.xsaves_enabled = guest_cpuid_has(vcpu, X86_FEATURE_XSAVE) &&
> > > > +                                 boot_cpu_has(X86_FEATURE_XSAVES);
> > >
> > > This looks very much like a functional change to SVM, which feels wrong
> > > for a patch with a subject of "KVM: VMX: Use wrmsr for switching between
> > > guest and host IA32_XSS" feels wrong.  Shouldn't this be unconditionally
> > > set false in this patch, and then enabled in " kvm: svm: Add support for
> > > XSAVES on AMD"?
> >
> > Nothing is being enabled here. Vcpu->arch.xsaves_enabled simply tells
> > us whether or not the guest can execute the XSAVES instruction. Any
> > guest with the ability to set CR4.OSXSAVE on an AMD host that supports
> > XSAVES can use the instruction.
>
> Not enabling per se, but it's a functional change as it means MSR_IA32_XSS
> will be written in kvm_load_{guest,host}_xsave_controls() if host_xss!=0.

Fortunately, host_xss is guaranteed to be zero for the nonce. :-)

Perhaps the commit message just needs to be updated? The only
alternatives I see are:
1. Deliberately introducing buggy code to be removed later in the series, or
2. Introducing SVM-specific code first, to be removed later in the series.