KVM: MMU: Optimize guest page table walk

This patch optimizes the guest page table walk by using get_user()
instead of copy_from_user().

With this patch applied, paging64_walk_addr_generic() has become
about 0.5us to 1.0us faster on my Phenom II machine with NPT on.

Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: Avi Kivity <avi@redhat.com>

1 files changed, 20 insertions, 3 deletions

diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
index 1b6899088f9..a32a1c80914 100644
--- a/arch/x86/kvm/paging_tmpl.h
+++ b/arch/x86/kvm/paging_tmpl.h
@@ -123,6 +123,7 @@ static int FNAME(walk_addr_generic)(struct guest_walker *walker,
                                     gva_t addr, u32 access)
 {
         pt_element_t pte;
+        pt_element_t __user *ptep_user;
         gfn_t table_gfn;
         unsigned index, pt_access, uninitialized_var(pte_access);
         gpa_t pte_gpa;
@@ -158,6 +159,9 @@ walk:
         pt_access = ACC_ALL;
 
         for (;;) {
+                gfn_t real_gfn;
+                unsigned long host_addr;
+
                 index = PT_INDEX(addr, walker->level);
 
                 table_gfn = gpte_to_gfn(pte);
@@ -166,9 +170,22 @@ walk:
                 walker->table_gfn[walker->level - 1] = table_gfn;
                 walker->pte_gpa[walker->level - 1] = pte_gpa;
 
-                if (kvm_read_guest_page_mmu(vcpu, mmu, table_gfn, &pte,
-                                            offset, sizeof(pte),
-                                            PFERR_USER_MASK|PFERR_WRITE_MASK)) {
+                real_gfn = mmu->translate_gpa(vcpu, gfn_to_gpa(table_gfn),
+                                              PFERR_USER_MASK|PFERR_WRITE_MASK);
+                if (real_gfn == UNMAPPED_GVA) {
+                        present = false;
+                        break;
+                }
+                real_gfn = gpa_to_gfn(real_gfn);
+
+                host_addr = gfn_to_hva(vcpu->kvm, real_gfn);
+                if (kvm_is_error_hva(host_addr)) {
+                        present = false;
+                        break;
+                }
+
+                ptep_user = (pt_element_t __user *)((void *)host_addr + offset);
+                if (get_user(pte, ptep_user)) {
                         present = false;
                         break;
                 }