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@xxxxxxxxxxxxxx>
---
arch/x86/kvm/vmx.c | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 3e556c68351b..e72b9660e51c 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -1659,8 +1659,14 @@ 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;
--
1.9.3
--
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