Subject: Re: [PATCH v13 51/85] KVM: VMX: Use __kvm_faultin_page() to get APIC access page/pfn

From: Sean Christopherson <[email-masked]>
Date: Mon, 21 Oct 2024 11:57:42 -0700

On Mon, Oct 21, 2024, Yan Zhao wrote:
> On Thu, Oct 10, 2024 at 11:23:53AM -0700, Sean Christopherson wrote:
> > Use __kvm_faultin_page() get the APIC access page so that KVM can
> > precisely release the refcounted page, i.e. to remove yet another user
> > of kvm_pfn_to_refcounted_page().  While the path isn't handling a guest
> > page fault, the semantics are effectively the same; KVM just happens to
> > be mapping the pfn into a VMCS field instead of a secondary MMU.
> > 
> > Tested-by: Alex Bennée <[email-masked]>
> > Signed-off-by: Sean Christopherson <[email-masked]>
> > ---

...

> > @@ -6838,10 +6840,13 @@ void vmx_set_apic_access_page_addr(struct kvm_vcpu *vcpu)
> >  		vmcs_write64(APIC_ACCESS_ADDR, pfn_to_hpa(pfn));
> >  
> >  	/*
> > -	 * Do not pin apic access page in memory, the MMU notifier
> > -	 * will call us again if it is migrated or swapped out.
> > +	 * Do not pin the APIC access page in memory so that it can be freely
> > +	 * migrated, the MMU notifier will call us again if it is migrated or
> > +	 * swapped out.  KVM backs the memslot with anonymous memory, the pfn
> > +	 * should always point at a refcounted page (if the pfn is valid).
> >  	 */
> > -	kvm_release_pfn_clean(pfn);
> > +	if (!WARN_ON_ONCE(!refcounted_page))
> > +		kvm_release_page_clean(refcounted_page);
> Why it's not
> if (!WARN_ON_ONCE(!refcounted_page)) {
> 	if (writable)
> 		kvm_release_page_dirty(refcounted_page)
> 	else
> 		kvm_release_page_clean(refcounted_page)
> }
> 
> or simply not pass "writable" to __kvm_faultin_pfn() as we know the slot is
> not read-only and then set dirty ?

__kvm_faultin_pfn() requires a non-NULL @writable.  The intent is to help ensure
the caller is actually checking whether a readable vs. writable mapping was
acquired.  For cases that explicitly pass FOLL_WRITE, it's awkward, but those
should be few and far between.

> if (!WARN_ON_ONCE(!refcounted_page))
> 	kvm_release_page_dirty(refcounted_page)

Ya, this is probably more correct?  Though I would strongly prefer to make any
change in behavior on top of this series.  The use of kvm_release_page_clean()
was added by commit 878940b33d76 ("KVM: VMX: Retry APIC-access page reload if
invalidation is in-progress"), and I suspect the only reason it added the
kvm_set_page_accessed() call is because there was no "unused" variant.  I.e. there
was no concious decision to set Accessed but not Dirty.