KVM: x86/mmu: fast_page_fault support for the TDP MMU

Make fast_page_fault interoperate with the TDP MMU by leveraging
walk_shadow_page_lockless_{begin,end} to acquire the RCU read lock and
introducing a new helper function kvm_tdp_mmu_fast_pf_get_last_sptep to
grab the lowest level sptep.

Suggested-by: Ben Gardon <bgardon@google.com>
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20210713220957.3493520-5-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index d5b0c8b..29010ab 100644 (file)
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3109,14 +3109,40 @@ static bool is_access_allowed(u32 fault_err_code, u64 spte)
 }
 
 /*
+ * Returns the last level spte pointer of the shadow page walk for the given
+ * gpa, and sets *spte to the spte value. This spte may be non-preset. If no
+ * walk could be performed, returns NULL and *spte does not contain valid data.
+ *
+ * Contract:
+ *  - Must be called between walk_shadow_page_lockless_{begin,end}.
+ *  - The returned sptep must not be used after walk_shadow_page_lockless_end.
+ */
+static u64 *fast_pf_get_last_sptep(struct kvm_vcpu *vcpu, gpa_t gpa, u64 *spte)
+{
+       struct kvm_shadow_walk_iterator iterator;
+       u64 old_spte;
+       u64 *sptep = NULL;
+
+       for_each_shadow_entry_lockless(vcpu, gpa, iterator, old_spte) {
+               sptep = iterator.sptep;
+               *spte = old_spte;
+
+               if (!is_shadow_present_pte(old_spte))
+                       break;
+       }
+
+       return sptep;
+}
+
+/*
  * Returns one of RET_PF_INVALID, RET_PF_FIXED or RET_PF_SPURIOUS.
  */
 static int fast_page_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u32 error_code)
 {
-       struct kvm_shadow_walk_iterator iterator;
        struct kvm_mmu_page *sp;
        int ret = RET_PF_INVALID;
        u64 spte = 0ull;
+       u64 *sptep = NULL;
        uint retry_count = 0;
 
        if (!page_fault_can_be_fast(error_code))
@@ -3127,14 +3153,15 @@ static int fast_page_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u32 error_code)
        do {
                u64 new_spte;
 
-               for_each_shadow_entry_lockless(vcpu, gpa, iterator, spte)
-                       if (!is_shadow_present_pte(spte))
-                               break;
+               if (is_tdp_mmu(vcpu->arch.mmu))
+                       sptep = kvm_tdp_mmu_fast_pf_get_last_sptep(vcpu, gpa, &spte);
+               else
+                       sptep = fast_pf_get_last_sptep(vcpu, gpa, &spte);
 
                if (!is_shadow_present_pte(spte))
                        break;
 
-               sp = sptep_to_sp(iterator.sptep);
+               sp = sptep_to_sp(sptep);
                if (!is_last_spte(spte, sp->role.level))
                        break;
 
@@ -3192,8 +3219,7 @@ static int fast_page_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u32 error_code)
                 * since the gfn is not stable for indirect shadow page. See
                 * Documentation/virt/kvm/locking.rst to get more detail.
                 */
-               if (fast_pf_fix_direct_spte(vcpu, sp, iterator.sptep, spte,
-                                           new_spte)) {
+               if (fast_pf_fix_direct_spte(vcpu, sp, sptep, spte, new_spte)) {
                        ret = RET_PF_FIXED;
                        break;
                }
@@ -3206,7 +3232,7 @@ static int fast_page_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u32 error_code)
 
        } while (true);
 
-       trace_fast_page_fault(vcpu, gpa, error_code, iterator.sptep, spte, ret);
+       trace_fast_page_fault(vcpu, gpa, error_code, sptep, spte, ret);
        walk_shadow_page_lockless_end(vcpu);
 
        return ret;
@@ -3841,11 +3867,9 @@ static int direct_page_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u32 error_code,
        if (page_fault_handle_page_track(vcpu, error_code, gfn))
                return RET_PF_EMULATE;
 
-       if (!is_tdp_mmu_fault) {
-               r = fast_page_fault(vcpu, gpa, error_code);
-               if (r != RET_PF_INVALID)
-                       return r;
-       }
+       r = fast_page_fault(vcpu, gpa, error_code);
+       if (r != RET_PF_INVALID)
+               return r;
 
        r = mmu_topup_memory_caches(vcpu, false);
        if (r)

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 228f0cc..88aff5d 100644 (file)
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -527,6 +527,10 @@ static inline bool tdp_mmu_set_spte_atomic_no_dirty_log(struct kvm *kvm,
        if (is_removed_spte(iter->old_spte))
                return false;
 
+       /*
+        * Note, fast_pf_fix_direct_spte() can also modify TDP MMU SPTEs and
+        * does not hold the mmu_lock.
+        */
        if (cmpxchg64(rcu_dereference(iter->sptep), iter->old_spte,
                      new_spte) != iter->old_spte)
                return false;
@@ -1536,3 +1540,40 @@ int kvm_tdp_mmu_get_walk(struct kvm_vcpu *vcpu, u64 addr, u64 *sptes,
 
        return leaf;
 }
+
+/*
+ * Returns the last level spte pointer of the shadow page walk for the given
+ * gpa, and sets *spte to the spte value. This spte may be non-preset. If no
+ * walk could be performed, returns NULL and *spte does not contain valid data.
+ *
+ * Contract:
+ *  - Must be called between kvm_tdp_mmu_walk_lockless_{begin,end}.
+ *  - The returned sptep must not be used after kvm_tdp_mmu_walk_lockless_end.
+ *
+ * WARNING: This function is only intended to be called during fast_page_fault.
+ */
+u64 *kvm_tdp_mmu_fast_pf_get_last_sptep(struct kvm_vcpu *vcpu, u64 addr,
+                                       u64 *spte)
+{
+       struct tdp_iter iter;
+       struct kvm_mmu *mmu = vcpu->arch.mmu;
+       gfn_t gfn = addr >> PAGE_SHIFT;
+       tdp_ptep_t sptep = NULL;
+
+       tdp_mmu_for_each_pte(iter, mmu, gfn, gfn + 1) {
+               *spte = iter.old_spte;
+               sptep = iter.sptep;
+       }
+
+       /*
+        * Perform the rcu_dereference to get the raw spte pointer value since
+        * we are passing it up to fast_page_fault, which is shared with the
+        * legacy MMU and thus does not retain the TDP MMU-specific __rcu
+        * annotation.
+        *
+        * This is safe since fast_page_fault obeys the contracts of this
+        * function as well as all TDP MMU contracts around modifying SPTEs
+        * outside of mmu_lock.
+        */
+       return rcu_dereference(sptep);
+}

diff --git a/arch/x86/kvm/mmu/tdp_mmu.h b/arch/x86/kvm/mmu/tdp_mmu.h
index 93e1bf5..361b47f 100644 (file)
--- a/arch/x86/kvm/mmu/tdp_mmu.h
+++ b/arch/x86/kvm/mmu/tdp_mmu.h
@@ -89,6 +89,8 @@ static inline void kvm_tdp_mmu_walk_lockless_end(void)
 
 int kvm_tdp_mmu_get_walk(struct kvm_vcpu *vcpu, u64 addr, u64 *sptes,
                         int *root_level);
+u64 *kvm_tdp_mmu_fast_pf_get_last_sptep(struct kvm_vcpu *vcpu, u64 addr,
+                                       u64 *spte);
 
 #ifdef CONFIG_X86_64
 bool kvm_mmu_init_tdp_mmu(struct kvm *kvm);