KVM: x86: use kvm_read_guest_page for emulator accesses

Emulator accesses are always done a page at a time, either by the emulator
itself (for fetches) or because we need to query the MMU for address
translations.  Speed up these accesses by using kvm_read_guest_page
and, in the case of fetches, by inlining kvm_read_guest_virt_helper and
dropping the loop around kvm_read_guest_page.

This final tweak saves 30-100 more clock cycles (4-10%), bringing the
count (as measured by kvm-unit-tests) down to 720-1100 clock cycles on
a Sandy Bridge Xeon host, compared to 2300-3200 before the whole series
and 925-1700 after the first two low-hanging fruit changes.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 19 insertions, 4 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 905edf8557e7..f750b69ca443 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4085,7 +4085,8 @@ static int kvm_read_guest_virt_helper(gva_t addr, void *val, unsigned int bytes,
 
                 if (gpa == UNMAPPED_GVA)
                         return X86EMUL_PROPAGATE_FAULT;
-                ret = kvm_read_guest(vcpu->kvm, gpa, data, toread);
+                ret = kvm_read_guest_page(vcpu->kvm, gpa >> PAGE_SHIFT, data,
+                                          offset, toread);
                 if (ret < 0) {
                         r = X86EMUL_IO_NEEDED;
                         goto out;
@@ -4106,10 +4107,24 @@ static int kvm_fetch_guest_virt(struct x86_emulate_ctxt *ctxt,
 {
         struct kvm_vcpu *vcpu = emul_to_vcpu(ctxt);
         u32 access = (kvm_x86_ops->get_cpl(vcpu) == 3) ? PFERR_USER_MASK : 0;
+        unsigned offset;
+        int ret;
 
-        return kvm_read_guest_virt_helper(addr, val, bytes, vcpu,
-                                          access | PFERR_FETCH_MASK,
-                                          exception);
+        /* Inline kvm_read_guest_virt_helper for speed.  */
+        gpa_t gpa = vcpu->arch.walk_mmu->gva_to_gpa(vcpu, addr, access|PFERR_FETCH_MASK,
+                                                    exception);
+        if (unlikely(gpa == UNMAPPED_GVA))
+                return X86EMUL_PROPAGATE_FAULT;
+
+        offset = addr & (PAGE_SIZE-1);
+        if (WARN_ON(offset + bytes > PAGE_SIZE))
+                bytes = (unsigned)PAGE_SIZE - offset;
+        ret = kvm_read_guest_page(vcpu->kvm, gpa >> PAGE_SHIFT, val,
+                                  offset, bytes);
+        if (unlikely(ret < 0))
+                return X86EMUL_IO_NEEDED;
+
+        return X86EMUL_CONTINUE;
 }
 
 int kvm_read_guest_virt(struct x86_emulate_ctxt *ctxt,