Re: Optimized clocksource with AMD AVIC enabled for Windows guest

[Vitaly Kuznetsov <vkuznets@redhat.com>]

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Paolo Bonzini <pbonzini@redhat.com> writes:

> On 04/02/21 13:24, Vitaly Kuznetsov wrote:
>> I checked Linux VMs on genuine Hyper-V and surprisingly
>> 'HV_DEPRECATING_AEOI_RECOMMENDED' is not exposed.
>
> Did the host have APICv/AVIC (and can Hyper-V use AVIC)?  AutoEOI is 
> still a useful optimization on hosts that don't have 
> hardware-accelerated EOI or interrupt injection.

I was under the impression that for Intel I need IvyBridge, I was
testing with Xeon E5-2420 v2. I don't have an AMD host with Hyper-V
handy so I spun a VM on Azure which has modern enough AMD EPYC 7452,
still no luck.

Surprisingly, Linux on KVM has code to handle AutoEOI recommendation
since 2017 (6c248aad81c89) so I assume it's possible to meet this bit in
the wild.

Anyway, I've smoke tested the attached patch (poorly tested and
hackish!) on Intel/AMD and WS2016 and nothing blew up
immediately. Kechen Lu, could you give it a spin in your 
environment? No userspace changes needed (will change if we decide to go
ahead with it).

-- 
Vitaly


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From cb129501199f1f3ab6f0ade81b11eb76d08b6b5b Mon Sep 17 00:00:00 2001
From: Vitaly Kuznetsov <vkuznets@redhat.com>
Date: Thu, 4 Feb 2021 13:31:41 +0100
Subject: [PATCH] KVM: x86: Deactivate APICv only when auto_eoi feature is in
 use

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
 arch/x86/include/asm/kvm_host.h |  3 +++
 arch/x86/kvm/cpuid.c            |  5 +++++
 arch/x86/kvm/hyperv.c           | 26 ++++++++++++++++++++------
 3 files changed, 28 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 3d6616f6f6ef..539fbb505d77 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -877,6 +877,9 @@ struct kvm_hv {
 	/* How many vCPUs have VP index != vCPU index */
 	atomic_t num_mismatched_vp_indexes;
 
+	/* How many SynICs use 'auto_eoi' feature */
+	atomic_t synic_auto_eoi_used;
+
 	struct hv_partition_assist_pg *hv_pa_pg;
 	struct kvm_hv_syndbg hv_syndbg;
 };
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 13036cf0b912..8df2dff37a5c 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -138,6 +138,11 @@ void kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu)
 		(best->eax & (1 << KVM_FEATURE_PV_UNHALT)))
 		best->eax &= ~(1 << KVM_FEATURE_PV_UNHALT);
 
+	/* Dirty hack: force HV_DEPRECATING_AEOI_RECOMMENDED. Not to be merged! */
+	best = kvm_find_cpuid_entry(vcpu, HYPERV_CPUID_ENLIGHTMENT_INFO, 0);
+	if (best)
+		best->eax |= HV_DEPRECATING_AEOI_RECOMMENDED;
+
 	if (!kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_MISC_ENABLE_NO_MWAIT)) {
 		best = kvm_find_cpuid_entry(vcpu, 0x1, 0);
 		if (best)
diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index 922c69dcca4d..7c9bc060889a 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -83,6 +83,11 @@ static bool synic_has_vector_auto_eoi(struct kvm_vcpu_hv_synic *synic,
 static void synic_update_vector(struct kvm_vcpu_hv_synic *synic,
 				int vector)
 {
+	struct kvm_vcpu *vcpu = synic_to_vcpu(synic);
+	struct kvm *kvm = vcpu->kvm;
+	struct kvm_hv *hv = &kvm->arch.hyperv;
+	int auto_eoi_old, auto_eoi_new;
+
 	if (vector < HV_SYNIC_FIRST_VALID_VECTOR)
 		return;
 
@@ -91,10 +96,25 @@ static void synic_update_vector(struct kvm_vcpu_hv_synic *synic,
 	else
 		__clear_bit(vector, synic->vec_bitmap);
 
+	auto_eoi_old = bitmap_weight(synic->auto_eoi_bitmap, 256);
+
 	if (synic_has_vector_auto_eoi(synic, vector))
 		__set_bit(vector, synic->auto_eoi_bitmap);
 	else
 		__clear_bit(vector, synic->auto_eoi_bitmap);
+
+	auto_eoi_new = bitmap_weight(synic->auto_eoi_bitmap, 256);
+
+	/* Hyper-V SynIC auto EOI SINT's are not compatible with APICV */
+	if (!auto_eoi_old && auto_eoi_new) {
+		if (atomic_inc_return(&hv->synic_auto_eoi_used) == 1)
+			kvm_request_apicv_update(vcpu->kvm, false,
+						 APICV_INHIBIT_REASON_HYPERV);
+	} else if (!auto_eoi_old && auto_eoi_new) {
+		if (atomic_dec_return(&hv->synic_auto_eoi_used) == 0)
+			kvm_request_apicv_update(vcpu->kvm, true,
+						 APICV_INHIBIT_REASON_HYPERV);
+	}
 }
 
 static int synic_set_sint(struct kvm_vcpu_hv_synic *synic, int sint,
@@ -903,12 +923,6 @@ int kvm_hv_activate_synic(struct kvm_vcpu *vcpu, bool dont_zero_synic_pages)
 {
 	struct kvm_vcpu_hv_synic *synic = vcpu_to_synic(vcpu);
 
-	/*
-	 * Hyper-V SynIC auto EOI SINT's are
-	 * not compatible with APICV, so request
-	 * to deactivate APICV permanently.
-	 */
-	kvm_request_apicv_update(vcpu->kvm, false, APICV_INHIBIT_REASON_HYPERV);
 	synic->active = true;
 	synic->dont_zero_synic_pages = dont_zero_synic_pages;
 	synic->control = HV_SYNIC_CONTROL_ENABLE;
-- 
2.29.2