Re: [PATCH v2 8/8] KVM: x86: hyper-v: Deactivate APICv only when AutoEOI feature is in use

[Sean Christopherson]

On Sun, Jul 18, 2021, Maxim Levitsky wrote:
> I am more inclined to fix this by just tracking if we hold the srcu
> lock on each VCPU manually, just as we track the srcu index anyway,
> and then kvm_request_apicv_update can use this to drop the srcu
> lock when needed.

The entire approach of dynamically adding/removing the memslot seems doomed to
failure, and is likely responsible for the performance issues with AVIC, e.g. a
single vCPU temporarily inhibiting AVIC will zap all SPTEs _twice_; on disable
and again on re-enable.

Rather than pile on more gunk, what about special casing the APIC access page
memslot in try_async_pf()?  E.g. zap the GFN in avic_update_access_page() when
disabling (and bounce through kvm_{inc,dec}_notifier_count()), and have the page
fault path skip directly to MMIO emulation without caching the MMIO info.  It'd
also give us a good excuse to rename try_async_pf() :-)

If lack of MMIO caching is a performance problem, an alternative solution would
be to allow caching but add a helper to zap the MMIO SPTE and request all vCPUs to
clear their cache.

It's all a bit gross, especially hijacking the mmu_notifier path, but IMO it'd be
less awful than the current memslot+SRCU mess.

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index f4d35289f59e..ea434d76cfb0 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3767,9 +3767,9 @@ static bool kvm_arch_setup_async_pf(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
                                  kvm_vcpu_gfn_to_hva(vcpu, gfn), &arch);
 }

-static bool try_async_pf(struct kvm_vcpu *vcpu, bool prefault, gfn_t gfn,
-                        gpa_t cr2_or_gpa, kvm_pfn_t *pfn, hva_t *hva,
-                        bool write, bool *writable)
+static bool kvm_faultin_pfn(struct kvm_vcpu *vcpu, bool prefault, gfn_t gfn,
+                           gpa_t cr2_or_gpa, kvm_pfn_t *pfn, hva_t *hva,
+                           bool write, bool *writable, int *r)
 {
        struct kvm_memory_slot *slot = kvm_vcpu_gfn_to_memslot(vcpu, gfn);
        bool async;
@@ -3780,13 +3780,26 @@ static bool try_async_pf(struct kvm_vcpu *vcpu, bool prefault, gfn_t gfn,
         * be zapped before KVM inserts a new MMIO SPTE for the gfn.
         */
        if (slot && (slot->flags & KVM_MEMSLOT_INVALID))
-               return true;
+               goto out_retry;

-       /* Don't expose private memslots to L2. */
-       if (is_guest_mode(vcpu) && !kvm_is_visible_memslot(slot)) {
-               *pfn = KVM_PFN_NOSLOT;
-               *writable = false;
-               return false;
+       if (!kvm_is_visible_memslot(slot)) {
+               /* Don't expose private memslots to L2. */
+               if (is_guest_mode(vcpu)) {
+                       *pfn = KVM_PFN_NOSLOT;
+                       *writable = false;
+                       return false;
+               }
+               /*
+                * If the APIC access page exists but is disabled, go directly
+                * to emulation without caching the MMIO access or creating a
+                * MMIO SPTE.  That way the cache doesn't need to be purged
+                * when the AVIC is re-enabled.
+                */
+               if (slot->id == APIC_ACCESS_PAGE_PRIVATE_MEMSLOT &&
+                   !vcpu->kvm->arch.apic_access_memslot_enabled) {
+                       *r = RET_PF_EMULATE;
+                       return true;
+               }
        }

        async = false;
@@ -3800,14 +3813,19 @@ static bool try_async_pf(struct kvm_vcpu *vcpu, bool prefault, gfn_t gfn,
                if (kvm_find_async_pf_gfn(vcpu, gfn)) {
                        trace_kvm_async_pf_doublefault(cr2_or_gpa, gfn);
                        kvm_make_request(KVM_REQ_APF_HALT, vcpu);
-                       return true;
-               } else if (kvm_arch_setup_async_pf(vcpu, cr2_or_gpa, gfn))
-                       return true;
+                       goto out_retry;
+               } else if (kvm_arch_setup_async_pf(vcpu, cr2_or_gpa, gfn)) {
+                       goto out_retry;
+               }
        }

        *pfn = __gfn_to_pfn_memslot(slot, gfn, false, NULL,
                                    write, writable, hva);
        return false;
+
+out_retry:
+       *r = RET_PF_RETRY;
+       return true;
 }

 static int direct_page_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u32 error_code,
@@ -3839,9 +3857,9 @@ static int direct_page_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u32 error_code,
        mmu_seq = vcpu->kvm->mmu_notifier_seq;
        smp_rmb();

-       if (try_async_pf(vcpu, prefault, gfn, gpa, &pfn, &hva,
-                        write, &map_writable))
-               return RET_PF_RETRY;
+       if (kvm_faultin_pfn(vcpu, prefault, gfn, gpa, &pfn, &hva, write,
+                           &map_writable, &r))
+               return r;

        if (handle_abnormal_pfn(vcpu, is_tdp ? 0 : gpa, gfn, pfn, ACC_ALL, &r))
                return r;
diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
index 490a028ddabe..9747124b877d 100644
--- a/arch/x86/kvm/mmu/paging_tmpl.h
+++ b/arch/x86/kvm/mmu/paging_tmpl.h
@@ -881,9 +881,9 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, gpa_t addr, u32 error_code,
        mmu_seq = vcpu->kvm->mmu_notifier_seq;
        smp_rmb();
 
-       if (try_async_pf(vcpu, prefault, walker.gfn, addr, &pfn, &hva,
-                        write_fault, &map_writable))
-               return RET_PF_RETRY;
+       if (kvm_faultin_pfn(vcpu, prefault, walker.gfn, addr, &pfn, &hva,
+                           write_fault, &map_writable, &r))
+               return r;
 
        if (handle_abnormal_pfn(vcpu, addr, walker.gfn, pfn, walker.pte_access, &r))
                return r;