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

[Yang, Weijiang]

On 5/16/2024 10:43 PM, Sean Christopherson wrote:
> 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.

Understood, thanks!