Re: [PATCH] KVM: x86/mmu: Conditionally call kvm_zap_obsolete_pages

[Sean Christopherson <seanjc@google.com>]

On Tue, Jul 30, 2024, Paolo Bonzini wrote:
> On 7/30/24 07:32, flyingpenghao@gmail.com wrote:
> > 
> > When tdp_mmu is enabled, invalid root calls kvm_tdp_mmu_zap_invalidated_roots
> > to implement it, and kvm_zap_obsolete_pages is not used.
> > 
> > Signed-off-by: Peng Hao<flyingpeng@tencent.com>
> > ---
> >   arch/x86/kvm/mmu/mmu.c | 3 ++-
> >   1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 901be9e420a4..e91586c2ef87 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -6447,7 +6447,8 @@ static void kvm_mmu_zap_all_fast(struct kvm *kvm)
> >   	 */
> >   	kvm_make_all_cpus_request(kvm, KVM_REQ_MMU_FREE_OBSOLETE_ROOTS);
> > -	kvm_zap_obsolete_pages(kvm);
> > +	if (!tdp_mmu_enabled)
> > +		kvm_zap_obsolete_pages(kvm);
> 
> Can't you have obsolete pages from the shadow MMU that's used for nested
> (nGPA->HPA) virtualization?

Yep.  And kvm_zap_obsolete_pages() is a relatively cheap nop if there are no
pages on active_mmu_pages.  E.g. we could check kvm_memslots_have_rmaps(), but I
don't see any point in doing so, as the existing code should be blazing fast
relative to the total cost of the zap.