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, 25 insertions, 0 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 64c42d90112b..2c9661f230a9 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5062,6 +5062,30 @@ void kvm_after_handle_nmi(struct kvm_vcpu *vcpu)
 }
 EXPORT_SYMBOL_GPL(kvm_after_handle_nmi);
 
+static void kvm_set_mmio_spte_mask(void)
+{
+	u64 mask;
+	int maxphyaddr = boot_cpu_data.x86_phys_bits;
+
+	/*
+	 * Set the reserved bits and the present bit of an paging-structure
+	 * entry to generate page fault with PFER.RSV = 1.
+	 */
+	mask = ((1ull << (62 - maxphyaddr + 1)) - 1) << maxphyaddr;
+	mask |= 1ull;
+
+#ifdef CONFIG_X86_64
+	/*
+	 * If reserved bit is not supported, clear the present bit to disable
+	 * mmio page fault.
+	 */
+	if (maxphyaddr == 52)
+		mask &= ~1ull;
+#endif
+
+	kvm_mmu_set_mmio_spte_mask(mask);
+}
+
 int kvm_arch_init(void *opaque)
 {
 	int r;
@@ -5088,6 +5112,7 @@ int kvm_arch_init(void *opaque)
 	if (r)
 		goto out;
 
+	kvm_set_mmio_spte_mask();
 	kvm_init_msr_list();
 
 	kvm_x86_ops = ops;