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

Yan Zhao
2024-10-21T10:25:33Z

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>
> ---
> arch/x86/kvm/vmx/vmx.c | 13 +++++++++----
> 1 file changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 851be0820e04..44cc25dfebba 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6790,8 +6790,10 @@ void vmx_set_apic_access_page_addr(struct kvm_vcpu *vcpu)
> struct kvm *kvm = vcpu->kvm;
> struct kvm_memslots *slots = kvm_memslots(kvm);
> struct kvm_memory_slot *slot;
> + struct page *refcounted_page;
> unsigned long mmu_seq;
> kvm_pfn_t pfn;
> + bool writable;
>
> /* Defer reload until vmcs01 is the current VMCS. */
> if (is_guest_mode(vcpu)) {
> @@ -6827,7 +6829,7 @@ void vmx_set_apic_access_page_addr(struct kvm_vcpu *vcpu)
> * controls the APIC-access page memslot, and only deletes the memslot
> * if APICv is permanently inhibited, i.e. the memslot won't reappear.
> */
> - pfn = gfn_to_pfn_memslot(slot, gfn);
> + pfn = __kvm_faultin_pfn(slot, gfn, FOLL_WRITE, &writable, &refcounted_page);
> if (is_error_noslot_pfn(pfn))
> return;
>
> @@ -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 ?

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

>
> /*
> * No need for a manual TLB flush at this point, KVM has already done a
> --
> 2.47.0.rc1.288.g06298d1525-goog
>