On 11/8/21 10:59, Kele Huang wrote:
Currently, AVIC is disabled if x2apic feature is exposed to guest
or in-kernel PIT is in re-injection mode.
We can enable AVIC with options:
Kmod args:
modprobe kvm_amd avic=1 nested=0 npt=1
QEMU args:
... -cpu host,-x2apic -global kvm-pit.lost_tick_policy=discard ...
When LAPIC works in xapic mode, both AVIC and PV_SEND_IPI feature
can accelerate IPI operations for guest. However, the relationship
between AVIC and PV_SEND_IPI feature is not sorted out.
In logical, AVIC accelerates most of frequently IPI operations
without VMM intervention, while the re-hooking of apic->send_IPI_xxx
from PV_SEND_IPI feature masks out it. People can get confused
if AVIC is enabled while getting lots of hypercall kvm_exits
from IPI.
In performance, benchmark tool
https://lore.kernel.org/kvm/20171219085010.4081-1-ynorov@xxxxxxxxxxxxxxxxxx/
shows below results:
Test env:
CPU: AMD EPYC 7742 64-Core Processor
2 vCPUs pinned 1:1
idle=poll
Test result (average ns per IPI of lots of running):
PV_SEND_IPI : 1860
AVIC : 1390
Besides, disscussions in https://lkml.org/lkml/2021/10/20/423
do have some solid performance test results to this.
This patch fixes this by masking out PV_SEND_IPI feature when
AVIC is enabled in setting up of guest vCPUs' CPUID.
Signed-off-by: Kele Huang <huangkele@xxxxxxxxxxxxx>
AVIC can change across migration. I think we should instead use a new
KVM_HINTS_* bit (KVM_HINTS_ACCELERATED_LAPIC or something like that).
The KVM_HINTS_* bits are intended to be changeable across migration,
even though we don't have for now anything equivalent to the Hyper-V
reenlightenment interrupt.
Paolo
---
arch/x86/kvm/cpuid.c | 4 ++--
arch/x86/kvm/svm/svm.c | 13 +++++++++++++
2 files changed, 15 insertions(+), 2 deletions(-)
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 2d70edb0f323..cc22975e2ac5 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -194,8 +194,6 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
best->ecx |= XFEATURE_MASK_FPSSE;
}
- kvm_update_pv_runtime(vcpu);
-
vcpu->arch.maxphyaddr = cpuid_query_maxphyaddr(vcpu);
vcpu->arch.reserved_gpa_bits = kvm_vcpu_reserved_gpa_bits_raw(vcpu);
@@ -208,6 +206,8 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
/* Invoke the vendor callback only after the above state is updated. */
static_call(kvm_x86_vcpu_after_set_cpuid)(vcpu);
+ kvm_update_pv_runtime(vcpu);
+
/*
* Except for the MMU, which needs to do its thing any vendor specific
* adjustments to the reserved GPA bits.
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index b36ca4e476c2..b13bcfb2617c 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -4114,6 +4114,19 @@ static void svm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
if (nested && guest_cpuid_has(vcpu, X86_FEATURE_SVM))
kvm_request_apicv_update(vcpu->kvm, false,
APICV_INHIBIT_REASON_NESTED);
+
+ if (!guest_cpuid_has(vcpu, X86_FEATURE_X2APIC) &&
+ !(nested && guest_cpuid_has(vcpu, X86_FEATURE_SVM))) {
+ /*
+ * PV_SEND_IPI feature masks out AVIC acceleration to IPI.
+ * So, we do not expose PV_SEND_IPI feature to guest when
+ * AVIC is enabled.
+ */
+ best = kvm_find_cpuid_entry(vcpu, KVM_CPUID_FEATURES, 0);
+ if (best && enable_apicv &&
+ (best->eax & (1 << KVM_FEATURE_PV_SEND_IPI)))
+ best->eax &= ~(1 << KVM_FEATURE_PV_SEND_IPI);
+ }
}
init_vmcb_after_set_cpuid(vcpu);
}
