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

[Maxim Levitsky]

On Tue, 2024-07-09 at 12:00 -0700, Sean Christopherson wrote:
> 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);


Makes sense, thanks!

Best regards,
	Maxim Levitsky