Re: [PATCH v2 29/49] KVM: x86: Remove unnecessary caching of KVM's PV CPUID base

[Sean Christopherson]

On Thu, Jul 04, 2024, Maxim Levitsky wrote:
> On Fri, 2024-05-17 at 10:39 -0700, Sean Christopherson wrote:
> > Now that KVM only searches for KVM's PV CPUID base when userspace sets
> > guest CPUID, drop the cache and simply do the search every time.
> > 
> > Practically speaking, this is a nop except for situations where userspace
> > sets CPUID _after_ running the vCPU, which is anything but a hot path,
> > e.g. QEMU does so only when hotplugging a vCPU.  And on the flip side,
> > caching guest CPUID information, especially information that is used to
> > query/modify _other_ CPUID state, is inherently dangerous as it's all too
> > easy to use stale information, i.e. KVM should only cache CPUID state when
> > the performance and/or programming benefits justify it.
> > 
> > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > ---

...

> > @@ -491,13 +479,6 @@ static int kvm_set_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2,
> >  	 * whether the supplied CPUID data is equal to what's already set.
> >  	 */
> >  	if (kvm_vcpu_has_run(vcpu)) {
> > -		/*
> > -		 * Note, runtime CPUID updates may consume other CPUID-driven
> > -		 * vCPU state, e.g. KVM or Xen CPUID bases.  Updating runtime
> > -		 * state before full CPUID processing is functionally correct
> > -		 * only because any change in CPUID is disallowed, i.e. using
> > -		 * stale data is ok because KVM will reject the change.
> > -		 */
> Hi,
> 
> Any reason why this comment was removed?

Because after this patch, runtime CPUID updates no longer consume other vCPU
state that is derived from guest CPUID.

> As I said earlier in the review.  It might make sense to replace this comment
> with a comment reflecting on why we need to call kvm_update_cpuid_runtime,
> that is solely to allow old == new compare to succeed.

Ya, I'll figure out a location and patch to document why KVM applies runtime
and quirks to the CPUID before checking.

> 
> >  		kvm_update_cpuid_runtime(vcpu);
> >  		kvm_apply_cpuid_pv_features_quirk(vcpu);