Re: [PATCH v10 24/27] KVM: x86: Enable CET virtualization for VMX and advertise to userspace

[Sean Christopherson <seanjc@google.com>]

On Thu, May 16, 2024, Weijiang Yang wrote:
> On 5/6/2024 5:41 PM, Yang, Weijiang wrote:
> > On 5/2/2024 7:34 AM, Sean Christopherson wrote:
> > > On Sun, Feb 18, 2024, Yang Weijiang wrote:
> > > > @@ -665,7 +665,7 @@ void kvm_set_cpu_caps(void)
> > > >           F(AVX512_VPOPCNTDQ) | F(UMIP) | F(AVX512_VBMI2) | F(GFNI) |
> > > >           F(VAES) | F(VPCLMULQDQ) | F(AVX512_VNNI) | F(AVX512_BITALG) |
> > > >           F(CLDEMOTE) | F(MOVDIRI) | F(MOVDIR64B) | 0 /*WAITPKG*/ |
> > > > -        F(SGX_LC) | F(BUS_LOCK_DETECT)
> > > > +        F(SGX_LC) | F(BUS_LOCK_DETECT) | F(SHSTK)
> > > >       );
> > > >       /* Set LA57 based on hardware capability. */
> > > >       if (cpuid_ecx(7) & F(LA57))
> > > > @@ -683,7 +683,8 @@ void kvm_set_cpu_caps(void)
> > > >           F(SPEC_CTRL_SSBD) | F(ARCH_CAPABILITIES) | F(INTEL_STIBP) |
> > > >           F(MD_CLEAR) | F(AVX512_VP2INTERSECT) | F(FSRM) |
> > > >           F(SERIALIZE) | F(TSXLDTRK) | F(AVX512_FP16) |
> > > > -        F(AMX_TILE) | F(AMX_INT8) | F(AMX_BF16) | F(FLUSH_L1D)
> > > > +        F(AMX_TILE) | F(AMX_INT8) | F(AMX_BF16) | F(FLUSH_L1D) |
> > > > +        F(IBT)
> > > >       );
> > > ...
> > > 
> > > > @@ -7977,6 +7993,18 @@ static __init void vmx_set_cpu_caps(void)
> > > >         if (cpu_has_vmx_waitpkg())
> > > >           kvm_cpu_cap_check_and_set(X86_FEATURE_WAITPKG);
> > > > +
> > > > +    /*
> > > > +     * Disable CET if unrestricted_guest is unsupported as KVM doesn't
> > > > +     * enforce CET HW behaviors in emulator. On platforms with
> > > > +     * VMX_BASIC[bit56] == 0, inject #CP at VMX entry with error code
> > > > +     * fails, so disable CET in this case too.
> > > > +     */
> > > > +    if (!cpu_has_load_cet_ctrl() || !enable_unrestricted_guest ||
> > > > +        !cpu_has_vmx_basic_no_hw_errcode()) {
> > > > +        kvm_cpu_cap_clear(X86_FEATURE_SHSTK);
> > > > +        kvm_cpu_cap_clear(X86_FEATURE_IBT);
> > > > +    }
> > > Oh!  Almost missed it.  This patch should explicitly kvm_cpu_cap_clear()
> > > X86_FEATURE_SHSTK and X86_FEATURE_IBT.  We *know* there are upcoming AMD CPUs
> > > that support at least SHSTK, so enumerating support for common code would yield
> > > a version of KVM that incorrectly advertises support for SHSTK.
> > > 
> > > I hope to land both Intel and AMD virtualization in the same kernel release, but
> > > there are no guarantees that will happen.  And explicitly clearing both SHSTK and
> > > IBT would guard against IBT showing up in some future AMD CPU in advance of KVM
> > > gaining full support.
> > 
> > Let me be clear on this, you want me to disable SHSTK/IBT with
> > kvm_cpu_cap_clear() unconditionally for now in this patch, and wait until
> > both AMD's SVM patches and this series are ready for guest CET, then remove
> > the disabling code in this patch for final merge, am I right?

No, allow it to be enabled for VMX, but explicitly disable it for SVM, i.e.

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 4aaffbf22531..b3df12af4ee6 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -5125,6 +5125,10 @@ static __init void svm_set_cpu_caps(void)
        kvm_caps.supported_perf_cap = 0;
        kvm_caps.supported_xss = 0;
 
+       /* KVM doesn't yet support CET virtualization for SVM. */
+       kvm_cpu_cap_clear(X86_FEATURE_SHSTK);
+       kvm_cpu_cap_clear(X86_FEATURE_IBT);
+
        /* CPUID 0x80000001 and 0x8000000A (SVM features) */
        if (nested) {
                kvm_cpu_cap_set(X86_FEATURE_SVM);

Then the SVM series can simply delete those lines when all is ready.