On Thu, 2021-01-28 at 13:43 +0100, Paolo Bonzini wrote:
> Independent of the answer to the above, this is really the only place
> where you're adding Xen code to a hot path. Can you please use a
> STATIC_KEY_FALSE kvm_has_xen_vcpu (and a static inline function) to
> quickly return from kvm_xen_has_interrupt() if no vCPU has a shared info
> set up?
Something like this, then?
From 6504c78f76efd8c60630959111bd77c28d43fca7 Mon Sep 17 00:00:00 2001
From: David Woodhouse <dwmw@xxxxxxxxxxxx>
Date: Fri, 29 Jan 2021 17:30:40 +0000
Subject: [PATCH] KVM: x86/xen: Add static branch to avoid overhead of checking
Xen upcall vector
Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
---
arch/x86/kvm/x86.c | 1 +
arch/x86/kvm/xen.c | 68 +++++++++++++++++++++++++++-------------------
arch/x86/kvm/xen.h | 15 +++++++++-
3 files changed, 55 insertions(+), 29 deletions(-)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 870dea74ea94..fb2bc362efd8 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -10580,6 +10580,7 @@ void kvm_arch_destroy_vm(struct kvm *kvm)
kvm_x86_ops.vm_destroy(kvm);
for (i = 0; i < kvm->arch.msr_filter.count; i++)
kfree(kvm->arch.msr_filter.ranges[i].bitmap);
+ kvm_xen_destroy(kvm);
kvm_pic_destroy(kvm);
kvm_ioapic_destroy(kvm);
kvm_free_vcpus(kvm);
diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index 3041f774493e..9b6766e5d84f 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -19,6 +19,8 @@
#include "trace.h"
+DEFINE_STATIC_KEY_DEFERRED_FALSE(kvm_xen_has_vector, HZ);
+
static int kvm_xen_shared_info_init(struct kvm *kvm, gfn_t gfn)
{
gpa_t gpa = gfn_to_gpa(gfn);
@@ -176,7 +178,7 @@ void kvm_xen_setup_runstate_page(struct kvm_vcpu *v)
kvm_xen_update_runstate(v, RUNSTATE_running, steal_time);
}
-int kvm_xen_has_interrupt(struct kvm_vcpu *v)
+int __kvm_xen_has_interrupt(struct kvm_vcpu *v)
{
u8 rc = 0;
@@ -184,37 +186,42 @@ int kvm_xen_has_interrupt(struct kvm_vcpu *v)
* If the global upcall vector (HVMIRQ_callback_vector) is set and
* the vCPU's evtchn_upcall_pending flag is set, the IRQ is pending.
*/
- if (v->arch.xen.vcpu_info_set && v->kvm->arch.xen.upcall_vector) {
- struct gfn_to_hva_cache *ghc = &v->arch.xen.vcpu_info_cache;
- struct kvm_memslots *slots = kvm_memslots(v->kvm);
- unsigned int offset = offsetof(struct vcpu_info, evtchn_upcall_pending);
-
- /* No need for compat handling here */
- BUILD_BUG_ON(offsetof(struct vcpu_info, evtchn_upcall_pending) !=
- offsetof(struct compat_vcpu_info, evtchn_upcall_pending));
- BUILD_BUG_ON(sizeof(rc) !=
- sizeof(((struct vcpu_info *)0)->evtchn_upcall_pending));
- BUILD_BUG_ON(sizeof(rc) !=
- sizeof(((struct compat_vcpu_info *)0)->evtchn_upcall_pending));
-
- /*
- * For efficiency, this mirrors the checks for using the valid
- * cache in kvm_read_guest_offset_cached(), but just uses
- * __get_user() instead. And falls back to the slow path.
- */
- if (likely(slots->generation == ghc->generation &&
- !kvm_is_error_hva(ghc->hva) && ghc->memslot)) {
- /* Fast path */
- __get_user(rc, (u8 __user *)ghc->hva + offset);
- } else {
- /* Slow path */
- kvm_read_guest_offset_cached(v->kvm, ghc, &rc, offset,
- sizeof(rc));
- }
+ struct gfn_to_hva_cache *ghc = &v->arch.xen.vcpu_info_cache;
+ struct kvm_memslots *slots = kvm_memslots(v->kvm);
+ unsigned int offset = offsetof(struct vcpu_info, evtchn_upcall_pending);
+
+ /* No need for compat handling here */
+ BUILD_BUG_ON(offsetof(struct vcpu_info, evtchn_upcall_pending) !=
+ offsetof(struct compat_vcpu_info, evtchn_upcall_pending));
+ BUILD_BUG_ON(sizeof(rc) !=
+ sizeof(((struct vcpu_info *)0)->evtchn_upcall_pending));
+ BUILD_BUG_ON(sizeof(rc) !=
+ sizeof(((struct compat_vcpu_info *)0)->evtchn_upcall_pending));
+
+ /*
+ * For efficiency, this mirrors the checks for using the valid
+ * cache in kvm_read_guest_offset_cached(), but just uses
+ * __get_user() instead. And falls back to the slow path.
+ */
+ if (likely(slots->generation == ghc->generation &&
+ !kvm_is_error_hva(ghc->hva) && ghc->memslot)) {
+ /* Fast path */
+ __get_user(rc, (u8 __user *)ghc->hva + offset);
+ } else {
+ /* Slow path */
+ kvm_read_guest_offset_cached(v->kvm, ghc, &rc, offset,
+ sizeof(rc));
}
+
return rc;
}
+void kvm_xen_destroy(struct kvm *kvm)
+{
+ if (kvm->arch.xen.upcall_vector)
+ static_branch_slow_dec_deferred(&kvm_xen_has_vector);
+}
+
int kvm_xen_hvm_set_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data)
{
struct kvm_vcpu *v;
@@ -300,6 +307,11 @@ int kvm_xen_hvm_set_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data)
break;
}
+ if (data->u.vector && !kvm->arch.xen.upcall_vector)
+ static_branch_inc(&kvm_xen_has_vector.key);
+ else if (kvm->arch.xen.upcall_vector && !data->u.vector)
+ static_branch_slow_dec_deferred(&kvm_xen_has_vector);
+
kvm->arch.xen.upcall_vector = data->u.vector;
r = 0;
break;
diff --git a/arch/x86/kvm/xen.h b/arch/x86/kvm/xen.h
index d64916ac4a12..d49cd8ea8da9 100644
--- a/arch/x86/kvm/xen.h
+++ b/arch/x86/kvm/xen.h
@@ -9,13 +9,16 @@
#ifndef __ARCH_X86_KVM_XEN_H__
#define __ARCH_X86_KVM_XEN_H__
+#include <linux/jump_label_ratelimit.h>
+
void kvm_xen_setup_runstate_page(struct kvm_vcpu *vcpu);
void kvm_xen_runstate_set_preempted(struct kvm_vcpu *vcpu);
-int kvm_xen_has_interrupt(struct kvm_vcpu *vcpu);
+int __kvm_xen_has_interrupt(struct kvm_vcpu *vcpu);
int kvm_xen_hvm_set_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data);
int kvm_xen_hvm_get_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data);
int kvm_xen_hypercall(struct kvm_vcpu *vcpu);
int kvm_xen_hvm_config(struct kvm_vcpu *vcpu, u64 data);
+void kvm_xen_destroy(struct kvm *kvm);
static inline bool kvm_xen_hypercall_enabled(struct kvm *kvm)
{
@@ -23,6 +26,16 @@ static inline bool kvm_xen_hypercall_enabled(struct kvm *kvm)
KVM_XEN_HVM_CONFIG_INTERCEPT_HCALL;
}
+extern struct static_key_false_deferred kvm_xen_has_vector;
+
+static inline int kvm_xen_has_interrupt(struct kvm_vcpu *vcpu)
+{
+ if (static_branch_unlikely(&kvm_xen_has_vector.key) &&
+ vcpu->arch.xen.vcpu_info_set && vcpu->kvm->arch.xen.upcall_vector)
+ return __kvm_xen_has_interrupt(vcpu);
+
+ return 0;
+}
/* 32-bit compatibility definitions, also used natively in 32-bit build */
#include <asm/pvclock-abi.h>
--
2.17.1
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