KVM: x86: retry non-page-table writing instructions

If the emulation is caused by #PF and it is non-page_table writing instruction,
it means the VM-EXIT is caused by shadow page protected, we can zap the shadow
page and retry this instruction directly

The idea is from Avi

Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

3 files changed, 71 insertions, 6 deletions

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index a10950a37928..8547958e3582 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -3702,6 +3702,11 @@ done:
         return (rc != X86EMUL_CONTINUE) ? EMULATION_FAILED : EMULATION_OK;
 }
 
+bool x86_page_table_writing_insn(struct x86_emulate_ctxt *ctxt)
+{
+        return ctxt->d & PageTable;
+}
+
 static bool string_insn_completed(struct x86_emulate_ctxt *ctxt)
 {
         /* The second termination condition only applies for REPE
diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 232c5a30ddc8..7a22eb81b4ca 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -1998,7 +1998,7 @@ void kvm_mmu_change_mmu_pages(struct kvm *kvm, unsigned int goal_nr_mmu_pages)
         kvm->arch.n_max_mmu_pages = goal_nr_mmu_pages;
 }
 
-static int kvm_mmu_unprotect_page(struct kvm *kvm, gfn_t gfn)
+int kvm_mmu_unprotect_page(struct kvm *kvm, gfn_t gfn)
 {
         struct kvm_mmu_page *sp;
         struct hlist_node *node;
@@ -2007,7 +2007,7 @@ static int kvm_mmu_unprotect_page(struct kvm *kvm, gfn_t gfn)
 
         pgprintk("%s: looking for gfn %llx\n", __func__, gfn);
         r = 0;
-
+        spin_lock(&kvm->mmu_lock);
         for_each_gfn_indirect_valid_sp(kvm, sp, gfn, node) {
                 pgprintk("%s: gfn %llx role %x\n", __func__, gfn,
                          sp->role.word);
@@ -2015,8 +2015,11 @@ static int kvm_mmu_unprotect_page(struct kvm *kvm, gfn_t gfn)
                 kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list);
         }
         kvm_mmu_commit_zap_page(kvm, &invalid_list);
+        spin_unlock(&kvm->mmu_lock);
+
         return r;
 }
+EXPORT_SYMBOL_GPL(kvm_mmu_unprotect_page);
 
 static void mmu_unshadow(struct kvm *kvm, gfn_t gfn)
 {
@@ -3698,9 +3701,8 @@ int kvm_mmu_unprotect_page_virt(struct kvm_vcpu *vcpu, gva_t gva)
 
         gpa = kvm_mmu_gva_to_gpa_read(vcpu, gva, NULL);
 
-        spin_lock(&vcpu->kvm->mmu_lock);
         r = kvm_mmu_unprotect_page(vcpu->kvm, gpa >> PAGE_SHIFT);
-        spin_unlock(&vcpu->kvm->mmu_lock);
+
         return r;
 }
 EXPORT_SYMBOL_GPL(kvm_mmu_unprotect_page_virt);
@@ -3721,10 +3723,18 @@ void __kvm_mmu_free_some_pages(struct kvm_vcpu *vcpu)
         kvm_mmu_commit_zap_page(vcpu->kvm, &invalid_list);
 }
 
+static bool is_mmio_page_fault(struct kvm_vcpu *vcpu, gva_t addr)
+{
+        if (vcpu->arch.mmu.direct_map || mmu_is_nested(vcpu))
+                return vcpu_match_mmio_gpa(vcpu, addr);
+
+        return vcpu_match_mmio_gva(vcpu, addr);
+}
+
 int kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gva_t cr2, u32 error_code,
                        void *insn, int insn_len)
 {
-        int r;
+        int r, emulation_type = EMULTYPE_RETRY;
         enum emulation_result er;
 
         r = vcpu->arch.mmu.page_fault(vcpu, cr2, error_code, false);
@@ -3736,7 +3746,10 @@ int kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gva_t cr2, u32 error_code,
                 goto out;
         }
 
-        er = x86_emulate_instruction(vcpu, cr2, 0, insn, insn_len);
+        if (is_mmio_page_fault(vcpu, cr2))
+                emulation_type = 0;
+
+        er = x86_emulate_instruction(vcpu, cr2, emulation_type, insn, insn_len);
 
         switch (er) {
         case EMULATE_DONE:
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index e24edbc7f2ec..7ba1ab73fd03 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4836,6 +4836,50 @@ static bool reexecute_instruction(struct kvm_vcpu *vcpu, gva_t gva)
         return false;
 }
 
+static bool retry_instruction(struct x86_emulate_ctxt *ctxt,
+                              unsigned long cr2,  int emulation_type)
+{
+        struct kvm_vcpu *vcpu = emul_to_vcpu(ctxt);
+        unsigned long last_retry_eip, last_retry_addr, gpa = cr2;
+
+        last_retry_eip = vcpu->arch.last_retry_eip;
+        last_retry_addr = vcpu->arch.last_retry_addr;
+
+        /*
+         * If the emulation is caused by #PF and it is non-page_table
+         * writing instruction, it means the VM-EXIT is caused by shadow
+         * page protected, we can zap the shadow page and retry this
+         * instruction directly.
+         *
+         * Note: if the guest uses a non-page-table modifying instruction
+         * on the PDE that points to the instruction, then we will unmap
+         * the instruction and go to an infinite loop. So, we cache the
+         * last retried eip and the last fault address, if we meet the eip
+         * and the address again, we can break out of the potential infinite
+         * loop.
+         */
+        vcpu->arch.last_retry_eip = vcpu->arch.last_retry_addr = 0;
+
+        if (!(emulation_type & EMULTYPE_RETRY))
+                return false;
+
+        if (x86_page_table_writing_insn(ctxt))
+                return false;
+
+        if (ctxt->eip == last_retry_eip && last_retry_addr == cr2)
+                return false;
+
+        vcpu->arch.last_retry_eip = ctxt->eip;
+        vcpu->arch.last_retry_addr = cr2;
+
+        if (!vcpu->arch.mmu.direct_map)
+                gpa = kvm_mmu_gva_to_gpa_write(vcpu, cr2, NULL);
+
+        kvm_mmu_unprotect_page(vcpu->kvm, gpa >> PAGE_SHIFT);
+
+        return true;
+}
+
 int x86_emulate_instruction(struct kvm_vcpu *vcpu,
                             unsigned long cr2,
                             int emulation_type,
@@ -4877,6 +4921,9 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
                 return EMULATE_DONE;
         }
 
+        if (retry_instruction(ctxt, cr2, emulation_type))
+                return EMULATE_DONE;
+
         /* this is needed for vmware backdoor interface to work since it
            changes registers values  during IO operation */
         if (vcpu->arch.emulate_regs_need_sync_from_vcpu) {