Service NMI requests after PMI requests in vcpu_enter_guest() so that KVM
does not need to cancel and redo the VM-Enter. Because APIC emulation
"injects" NMIs via KVM_REQ_NMI, handling PMI requests after NMI requests
means KVM won't detect the pending NMI request until the final check for
outstanding requests. Detecting requests at the final stage is costly as
KVM has already loaded guest state, potentially queued events for
injection, disabled IRQs, dropped SRCU, etc., most of which needs to be
unwound.
Move the code snippet processing KVM_REQ_NMI to the one after processing
KVM_REQ_PMI to make KVM vPMU more efficient. Note that changing the order
of the code snippets does make any functional change to KVM. KVM allows two
pending NMIs while x86 architecture allows only one pending. This is
because KVM may not be able to process NMI quickly. Therefore, if a PMI
generated NMI is coincident two other NMIs pending (or one other pending
NMI and KVM blocks NMI) in KVM, then it will be dropped and it is
architecturely ok, since x86 CPU does not guarantee the delivery of each
NMI.
Using SPEC2017 benchmark programs running along with Intel vtune in a VM
demonstrates that the following code change reduces 800~1500 canceled
VM-Enters per second.
Some glory details:
Probe the invocation to vmx_cancel_injection():
$ perf probe -a vmx_cancel_injection
$ perf stat -a -e probe:vmx_cancel_injection -I 10000 # per 10 seconds
Partial results when SPEC2017 with Intel vtune are running in the VM:
On kernel without the change:
10.010018010 14254 probe:vmx_cancel_injection
20.037646388 15207 probe:vmx_cancel_injection
30.078739816 15261 probe:vmx_cancel_injection
40.114033258 15085 probe:vmx_cancel_injection
50.149297460 15112 probe:vmx_cancel_injection
60.185103088 15104 probe:vmx_cancel_injection
On kernel with the change:
10.003595390 40 probe:vmx_cancel_injection
20.017855682 31 probe:vmx_cancel_injection
30.028355883 34 probe:vmx_cancel_injection
40.038686298 31 probe:vmx_cancel_injection
50.048795162 20 probe:vmx_cancel_injection
60.069057747 19 probe:vmx_cancel_injection
v1: https://lore.kernel.org/all/20230927040939.342643-1-mizhang@xxxxxxxxxx/
Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Signed-off-by: Mingwei Zhang <mizhang@xxxxxxxxxx>
---
arch/x86/kvm/x86.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 42a4e8f5e89a..302b6f8ddfb1 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -10580,12 +10580,12 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
if (kvm_check_request(KVM_REQ_SMI, vcpu))
process_smi(vcpu);
#endif
- if (kvm_check_request(KVM_REQ_NMI, vcpu))
- process_nmi(vcpu);
if (kvm_check_request(KVM_REQ_PMU, vcpu))
kvm_pmu_handle_event(vcpu);
if (kvm_check_request(KVM_REQ_PMI, vcpu))
kvm_pmu_deliver_pmi(vcpu);
+ if (kvm_check_request(KVM_REQ_NMI, vcpu))
+ process_nmi(vcpu);
if (kvm_check_request(KVM_REQ_IOAPIC_EOI_EXIT, vcpu)) {
BUG_ON(vcpu->arch.pending_ioapic_eoi > 255);
if (test_bit(vcpu->arch.pending_ioapic_eoi,
base-commit: 73554b29bd70546c1a9efc9c160641ef1b849358
--
2.42.0.582.g8ccd20d70d-goog
