Re: [PATCH] KVM: x86: Move kvm_check_request(KVM_REQ_NMI) after kvm_check_request(KVM_REQ_NMI)
From: Xin Li
Date: Wed Sep 27 2023 - 01:19:46 EST
On 9/26/2023 9:15 PM, Mingwei Zhang wrote:
ah, typo in the subject: The 2nd KVM_REQ_NMI should be KVM_REQ_PMI.
Sorry about that.
On Tue, Sep 26, 2023 at 9:09 PM Mingwei Zhang <mizhang@xxxxxxxxxx> wrote:
Move kvm_check_request(KVM_REQ_NMI) after kvm_check_request(KVM_REQ_NMI).
Please remove it, no need to repeat the subject.
When vPMU is active use, processing each KVM_REQ_PMI will generate a
KVM_REQ_NMI. Existing control flow after KVM_REQ_PMI finished will fail the
guest enter, jump to kvm_x86_cancel_injection(), and re-enter
vcpu_enter_guest(), this wasted lot of cycles and increase the overhead for
vPMU as well as the virtualization.
Optimization is after correctness, so please explain if this is correct
first!
So move the code snippet of kvm_check_request(KVM_REQ_NMI) to make KVM
runloop more efficient with vPMU.
To evaluate the effectiveness of this change, we launch a 8-vcpu QEMU VM on
an Intel SPR CPU. In the VM, we run perf with all 48 events Intel vtune
uses. In addition, we use SPEC2017 benchmark programs as the workload with
the setup of using single core, single thread.
At the host level, we probe the invocations to vmx_cancel_injection() with
the following command:
$ perf probe -a vmx_cancel_injection
$ perf stat -a -e probe:vmx_cancel_injection -I 10000 # per 10 seconds
The following is the result that we collected at beginning of the spec2017
benchmark run (so mostly for 500.perlbench_r in spec2017). Kindly forgive
the incompleteness.
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
From the above, it is clear that we save 1500 invocations per vcpu per
second to vmx_cancel_injection() for workloads like perlbench.
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
--
2.42.0.515.g380fc7ccd1-goog
--
Thanks!
Xin