x86, kvm, vmx: Always use LOAD_IA32_EFER if available

At least on Sandy Bridge, letting the CPU switch IA32_EFER is much
faster than switching it manually.

I benchmarked this using the vmexit kvm-unit-test (single run, but
GOAL multiplied by 5 to do more iterations):

Test                                  Before      After    Change
cpuid                                   2000       1932    -3.40%
vmcall                                  1914       1817    -5.07%
mov_from_cr8                              13         13     0.00%
mov_to_cr8                                19         19     0.00%
inl_from_pmtimer                       19164      10619   -44.59%
inl_from_qemu                          15662      10302   -34.22%
inl_from_kernel                         3916       3802    -2.91%
outl_to_kernel                          2230       2194    -1.61%
mov_dr                                   172        176     2.33%
ipi                                (skipped)  (skipped)
ipi+halt                           (skipped)  (skipped)
ple-round-robin                           13         13     0.00%
wr_tsc_adjust_msr                       1920       1845    -3.91%
rd_tsc_adjust_msr                       1892       1814    -4.12%
mmio-no-eventfd:pci-mem                16394      11165   -31.90%
mmio-wildcard-eventfd:pci-mem           4607       4645     0.82%
mmio-datamatch-eventfd:pci-mem          4601       4610     0.20%
portio-no-eventfd:pci-io               11507       7942   -30.98%
portio-wildcard-eventfd:pci-io          2239       2225    -0.63%
portio-datamatch-eventfd:pci-io         2250       2234    -0.71%

I haven't explicitly computed the significance of these numbers,
but this isn't subtle.
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
[The results were reproducible on all of Nehalem, Sandy Bridge and
 Ivy Bridge.  The slowness of manual switching is because writing
 to EFER with WRMSR triggers a TLB flush, even if the only bit you're
 touching is SCE (so the page table format is not affected).  Doing
 the write as part of vmentry/vmexit, instead, does not flush the TLB,
 probably because all processors that have EPT also have VPID. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx.c

@@ -1662,8 +1662,14 @@ static bool update_transition_efer(struct vcpu_vmx *vmx, int efer_offset)
 	vmx->guest_msrs[efer_offset].mask = ~ignore_bits;
 
 	clear_atomic_switch_msr(vmx, MSR_EFER);
-	/* On ept, can't emulate nx, and must switch nx atomically */
-	if (enable_ept && ((vmx->vcpu.arch.efer ^ host_efer) & EFER_NX)) {
+
+	/*
+	 * On EPT, we can't emulate NX, so we must switch EFER atomically.
+	 * On CPUs that support "load IA32_EFER", always switch EFER
+	 * atomically, since it's faster than switching it manually.
+	 */
+	if (cpu_has_load_ia32_efer ||
+	    (enable_ept && ((vmx->vcpu.arch.efer ^ host_efer) & EFER_NX))) {
 		guest_efer = vmx->vcpu.arch.efer;
 		if (!(guest_efer & EFER_LMA))
 			guest_efer &= ~EFER_LME;