Subject: Re: [PATCH v3 10/25] KVM: x86/mmu: Drop the return code from kvm_x86_ops.remove_external_spte()

From: Sean Christopherson <[email-masked]>
Date: Wed, 22 Oct 2025 12:08:45 -0700

On Wed, Oct 22, 2025, Yan Zhao wrote:
> On Thu, Oct 16, 2025 at 05:32:28PM -0700, Sean Christopherson wrote:
> > Opportunistically pass the spte instead of the pfn, as the API is clearly
> > about removing an spte.
> >From my perspective, "remove_external_spte" means removing an external SPTE (not
> a mirror SPTE). So passing in pfn_for_gfn seems reasonable as well.
> 
> Additionally, passing in the pfn eliminates potential concerns about incorrect
> spte content.

No, it just makes bugs harder to debug.  E.g. it doesn't magically guarantee the
@pfn matches the pfn that was mapped into the S-EPT.

> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index 48598d017d6f..7e92aebd07e8 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -1855,8 +1855,8 @@ struct kvm_x86_ops {
> >  				 void *external_spt);
> >  
> >  	/* Update external page table from spte getting removed, and flush TLB. */
> > -	int (*remove_external_spte)(struct kvm *kvm, gfn_t gfn, enum pg_level level,
> > -				    kvm_pfn_t pfn_for_gfn);
> > +	void (*remove_external_spte)(struct kvm *kvm, gfn_t gfn, enum pg_level level,
> > +				     u64 spte);

Thinking more about what "spte" actually tracks, I think I'll rename it to
"mirror_spte".

> Also update set_external_spte?

Ooh, yeah, good call.  And we can use the mirror_spte information to assert that
KVM expects full RWX permissions, e.g. that we aren't creation a security hole by
letting the guest write memory that KVM thinks is read-only (extreme paranoia,
more for documentation purposes).