On 18/05/21 16:43, Vitaly Kuznetsov wrote:
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: in case APICv/AVIC usage is impossible, AutoEOI
is still preferred.
Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
Should it also disable APICv unconditionally if
HV_DEPRECATING_AEOI_RECOMMENDED is not in the guest CPUID? That should
avoid ping-pong between enabled and disabled APICv even in pathological
cases that we cannot think about.
Paolo
---
arch/x86/include/asm/kvm_host.h | 3 +++
arch/x86/kvm/hyperv.c | 27 +++++++++++++++++++++------
2 files changed, 24 insertions(+), 6 deletions(-)
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index bf5807d35339..5e03ab4c0e4f 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -936,6 +936,9 @@ struct kvm_hv {
/* How many vCPUs have VP index != vCPU index */
atomic_t num_mismatched_vp_indexes;
+ /* How many SynICs use 'AutoEOI' 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/hyperv.c b/arch/x86/kvm/hyperv.c
index f98370a39936..89e7d5b99279 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -87,6 +87,10 @@ 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;
@@ -95,10 +99,25 @@ 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 SINTs 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_new && auto_eoi_old) {
+ 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,
@@ -931,12 +950,6 @@ 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;
@@ -2198,6 +2211,8 @@ 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;
+ if (enable_apicv)
+ ent->eax |= HV_DEPRECATING_AEOI_RECOMMENDED;
/*
* Default number of spinlock retry attempts, matches
* HyperV 2016.
