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

[Yang, Weijiang]

On 5/2/2024 7:15 AM, Sean Christopherson wrote:
> On Sun, Feb 18, 2024, Yang Weijiang wrote:
> > @@ -696,6 +697,20 @@ void kvm_set_cpu_caps(void)
> >   		kvm_cpu_cap_set(X86_FEATURE_INTEL_STIBP);
> >   	if (boot_cpu_has(X86_FEATURE_AMD_SSBD))
> >   		kvm_cpu_cap_set(X86_FEATURE_SPEC_CTRL_SSBD);
> > +	/*
> > +	 * Don't use boot_cpu_has() to check availability of IBT because the
> > +	 * feature bit is cleared in boot_cpu_data when ibt=off is applied
> > +	 * in host cmdline.
> I'm not convinced this is a good reason to diverge from the host kernel.  E.g.
> PCID and many other features honor the host setup, I don't see what makes IBT
> special.
Hi, Sean,
We synced the issue internally, and got conclusion that KVM should honor host IBT config.
In this case IBT bit in boot_cpu_data should be honored.  With this policy, it can avoid CPUID
confusion to guest side due to host ibt=off config. Host side xstate support couldn't be an issue
because we already have below check in this patch:

+ if ((kvm_caps.supported_xss & (XFEATURE_MASK_CET_USER | + XFEATURE_MASK_CET_KERNEL)) != + (XFEATURE_MASK_CET_USER | XFEATURE_MASK_CET_KERNEL)) { + kvm_cpu_cap_clear(X86_FEATURE_SHSTK); + kvm_cpu_cap_clear(X86_FEATURE_IBT); + kvm_caps.supported_xss &= ~(XFEATURE_MASK_CET_USER | + XFEATURE_MASK_CET_KERNEL); + }

What's your thoughts? Should I just remove the quirk here and keep everything normal and
peaceful?