KVM: MMU: Optimize pte permission checks

walk_addr_generic() permission checks are a maze of branchy code, which is
performed four times per lookup.  It depends on the type of access, efer.nxe,
cr0.wp, cr4.smep, and in the near future, cr4.smap.

Optimize this away by precalculating all variants and storing them in a
bitmap.  The bitmap is recalculated when rarely-changing variables change
(cr0, cr4) and is indexed by the often-changing variables (page fault error
code, pte access permissions).

The permission check is moved to the end of the loop, otherwise an SMEP
fault could be reported as a false positive, when PDE.U=1 but PTE.U=0.
Noted by Xiao Guangrong.

The result is short, branch-free code.

Reviewed-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

1 files changed, 4 insertions, 7 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 19047eafa38d..497226e49d4b 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3672,20 +3672,17 @@ static int vcpu_mmio_gva_to_gpa(struct kvm_vcpu *vcpu, unsigned long gva,
                                 gpa_t *gpa, struct x86_exception *exception,
                                 bool write)
 {
-        u32 access = (kvm_x86_ops->get_cpl(vcpu) == 3) ? PFERR_USER_MASK : 0;
+        u32 access = ((kvm_x86_ops->get_cpl(vcpu) == 3) ? PFERR_USER_MASK : 0)
+                | (write ? PFERR_WRITE_MASK : 0);
 
-        if (vcpu_match_mmio_gva(vcpu, gva) &&
-                  check_write_user_access(vcpu, write, access,
-                  vcpu->arch.access)) {
+        if (vcpu_match_mmio_gva(vcpu, gva)
+            && !permission_fault(vcpu->arch.walk_mmu, vcpu->arch.access, access)) {
                 *gpa = vcpu->arch.mmio_gfn << PAGE_SHIFT |
                                         (gva & (PAGE_SIZE - 1));
                 trace_vcpu_match_mmio(gva, *gpa, write, false);
                 return 1;
         }
 
-        if (write)
-                access |= PFERR_WRITE_MASK;
-
         *gpa = vcpu->arch.walk_mmu->gva_to_gpa(vcpu, gva, access, exception);
 
         if (*gpa == UNMAPPED_GVA)