On 8/27/2025 3:24 PM, Xin Li wrote:

On 8/26/2025 3:17 PM, Sean Christopherson wrote:

quoted

quoted

+        if (!kvm_cpu_cap_has(X86_FEATURE_SHSTK))
+            wrmsrns(MSR_IA32_FRED_SSP0, vmx->msr_guest_fred_ssp0);

FWIW, if we can't get an SDM change, don't bother with RDMSR/WRMSRNS, just
configure KVM to intercept accesses.  Then in kvm_set_msr_common(), pivot on
X86_FEATURE_SHSTK, e.g.

Intercepting is a solid approach: it ensures the guest value is fully
virtual and does not affect the hardware FRED SSP0 MSR.  Of course the code
is also simplified.

quoted

    case MSR_IA32_U_CET:
    case MSR_IA32_PL0_SSP ... MSR_IA32_PL3_SSP:
        if (!kvm_cpu_cap_has(X86_FEATURE_SHSTK)) {
            WARN_ON_ONCE(msr != MSR_IA32_FRED_SSP0);
            vcpu->arch.fred_rsp0_fallback = data;

Putting fred_rsp0_fallback in struct kvm_vcpu_arch reminds me one thing:

We know AMD will do FRED and follow the FRED spec for bare metal, but
regarding virtualization of FRED, I have no idea how it will be done on
AMD, so I keep the KVM FRED code in VMX files, e.g., msr_guest_fred_rsp0 is
defined in struct vcpu_vmx, and saved/restored in vmx.c.

It is a future task to make common KVM FRED code for Intel and AMD.