[PATCH v4 11/16] KVM: x86: hyper-v: Deactivate APICv only when AutoEOI feature is in use

Maxim Levitsky
2021-08-10T20:54:05Z

From: Vitaly Kuznetsov <vkuznets@redhat.com>

APICV_INHIBIT_REASON_HYPERV is currently unconditionally forced upon
SynIC activation as SynIC's AutoEOI is incompatible with APICv/AVIC. It is,
however, possible to track whether the feature was actually used by the
guest and only inhibit APICv/AVIC when needed.

TLFS suggests a dedicated 'HV_DEPRECATING_AEOI_RECOMMENDED' flag to let
Windows know that AutoEOI feature should be avoided. While it's up to
KVM userspace to set the flag, KVM can help a bit by exposing global
APICv/AVIC enablement.

Maxim:
- always set HV_DEPRECATING_AEOI_RECOMMENDED in kvm_get_hv_cpuid,
since this feature can be used regardless of AVIC

Paolo:
- use arch.apicv_update_lock to protect the hv->synic_auto_eoi_used
instead of atomic ops

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
---
arch/x86/include/asm/kvm_host.h | 6 ++++++
arch/x86/kvm/hyperv.c | 32 ++++++++++++++++++++++++++------
2 files changed, 32 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 27627e32f362..a0ddff8d13c4 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -982,6 +982,12 @@ struct kvm_hv {
/* How many vCPUs have VP index != vCPU index */
atomic_t num_mismatched_vp_indexes;

+ /*
+ * How many SynICs use 'AutoEOI' feature
+ * (protected by arch.apicv_update_lock)
+ */
+ unsigned int synic_auto_eoi_used;
+
struct hv_partition_assist_pg *hv_pa_pg;
struct kvm_hv_syndbg hv_syndbg;
};
diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index b07592ca92f0..031ea0fb3e2f 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -88,6 +88,10 @@ 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 = hv_synic_to_vcpu(synic);
+ struct kvm_hv *hv = to_kvm_hv(vcpu->kvm);
+ int auto_eoi_old, auto_eoi_new;
+
if (vector < HV_SYNIC_FIRST_VALID_VECTOR)
return;

@@ -96,10 +100,30 @@ 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);
+
+ if (!!auto_eoi_old == !!auto_eoi_new)
+ return;
+
+ mutex_lock(&vcpu->kvm->arch.apicv_update_lock);
+
+ if (auto_eoi_new)
+ hv->synic_auto_eoi_used++;
+ else
+ hv->synic_auto_eoi_used--;
+
+ __kvm_request_apicv_update(vcpu->kvm,
+ !hv->synic_auto_eoi_used,
+ APICV_INHIBIT_REASON_HYPERV);
+
+ mutex_unlock(&vcpu->kvm->arch.apicv_update_lock);
}

static int synic_set_sint(struct kvm_vcpu_hv_synic *synic, int sint,
@@ -933,12 +957,6 @@ int kvm_hv_activate_synic(struct kvm_vcpu *vcpu, bool dont_zero_synic_pages)

synic = to_hv_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;
@@ -2466,6 +2484,8 @@ int kvm_get_hv_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid2 *cpuid,
ent->eax |= HV_X64_ENLIGHTENED_VMCS_RECOMMENDED;
if (!cpu_smt_possible())
ent->eax |= HV_X64_NO_NONARCH_CORESHARING;
+
+ ent->eax |= HV_DEPRECATING_AEOI_RECOMMENDED;
/*
* Default number of spinlock retry attempts, matches
* HyperV 2016.
--
2.26.3