KVM: MMU: mmio page fault support

The idea is from Avi:

| We could cache the result of a miss in an spte by using a reserved bit, and
| checking the page fault error code (or seeing if we get an ept violation or
| ept misconfiguration), so if we get repeated mmio on a page, we don't need to
| search the slot list/tree.
| (https://lkml.org/lkml/2011/2/22/221)

When the page fault is caused by mmio, we cache the info in the shadow page
table, and also set the reserved bits in the shadow page table, so if the mmio
is caused again, we can quickly identify it and emulate it directly

Searching mmio gfn in memslots is heavy since we need to walk all memeslots, it
can be reduced by this feature, and also avoid walking guest page table for
soft mmu.

[jan: fix operator precedence issue]

Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

1 files changed, 21 insertions, 1 deletions

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index a644acb6ed80..e65a158dee64 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -3594,6 +3594,17 @@ static u32 vmx_secondary_exec_control(struct vcpu_vmx *vmx)
         return exec_control;
 }
 
+static void ept_set_mmio_spte_mask(void)
+{
+        /*
+         * EPT Misconfigurations can be generated if the value of bits 2:0
+         * of an EPT paging-structure entry is 110b (write/execute).
+         * Also, magic bits (0xffull << 49) is set to quickly identify mmio
+         * spte.
+         */
+        kvm_mmu_set_mmio_spte_mask(0xffull << 49 | 0x6ull);
+}
+
 /*
  * Sets up the vmcs for emulated real mode.
  */
@@ -4671,11 +4682,19 @@ static void ept_misconfig_inspect_spte(struct kvm_vcpu *vcpu, u64 spte,
 static int handle_ept_misconfig(struct kvm_vcpu *vcpu)
 {
         u64 sptes[4];
-        int nr_sptes, i;
+        int nr_sptes, i, ret;
         gpa_t gpa;
 
         gpa = vmcs_read64(GUEST_PHYSICAL_ADDRESS);
 
+        ret = handle_mmio_page_fault_common(vcpu, gpa, true);
+        if (likely(ret == 1))
+                return x86_emulate_instruction(vcpu, gpa, 0, NULL, 0) ==
+                                              EMULATE_DONE;
+        if (unlikely(!ret))
+                return 1;
+
+        /* It is the real ept misconfig */
         printk(KERN_ERR "EPT: Misconfiguration.\n");
         printk(KERN_ERR "EPT: GPA: 0x%llx\n", gpa);
 
@@ -7102,6 +7121,7 @@ static int __init vmx_init(void)
         if (enable_ept) {
                 kvm_mmu_set_mask_ptes(0ull, 0ull, 0ull, 0ull,
                                 VMX_EPT_EXECUTABLE_MASK);
+                ept_set_mmio_spte_mask();
                 kvm_enable_tdp();
         } else
                 kvm_disable_tdp();