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

Sean Christopherson
2024-10-21T18:57:46Z

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 <alex.bennee@linaro.org>
> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > ---

...

> > @@ -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.