Re: Optimized clocksource with AMD AVIC enabled for Windows guest

[Vitaly Kuznetsov]

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

>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