diff mbox series

[09/10] KVM: x86: Kill KVM_REQ_GLOBAL_CLOCK_UPDATE

Message ID 20240418193528.41780-10-dwmw2@infradead.org (mailing list archive)
State New
Headers show
Series Cleaning up the KVM clock mess | expand

Commit Message

David Woodhouse April 18, 2024, 7:34 p.m. UTC 
From: David Woodhouse <dwmw@amazon.co.uk>

This was introduced in commit 0061d53daf26 ("KVM: x86: limit difference
between kvmclock updates") to reduce cross-vCPU differences which arose
because the KVM clock was based on CLOCK_MONOTONIC and thus subject to
NTP frequency corrections.

However, commit 53fafdbb8b21 ("KVM: x86: switch KVMCLOCK base to
monotonic raw clock") switched to using CLOCK_MONOTONIC_RAW as the basis
for the KVM clock, avoiding the NTP frequency skew altogether.

So remove KVM_REQ_GLOBAL_CLOCK_UPDATE. In kvm_write_system_time(), all
that's needed is a single KVM_REQ_CLOCK_UPDATE for the vCPU whose KVM
clock is being configured.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
---
 arch/x86/include/asm/kvm_host.h |  2 +-
 arch/x86/kvm/x86.c              | 57 ++-------------------------------
 2 files changed, 4 insertions(+), 55 deletions(-)

Comments

Durrant, Paul April 19, 2024, 3:57 p.m. UTC | #1 
On 18/04/2024 20:34, David Woodhouse wrote:
> From: David Woodhouse <dwmw@amazon.co.uk>
> 
> This was introduced in commit 0061d53daf26 ("KVM: x86: limit difference
> between kvmclock updates") to reduce cross-vCPU differences which arose
> because the KVM clock was based on CLOCK_MONOTONIC and thus subject to
> NTP frequency corrections.
> 
> However, commit 53fafdbb8b21 ("KVM: x86: switch KVMCLOCK base to
> monotonic raw clock") switched to using CLOCK_MONOTONIC_RAW as the basis
> for the KVM clock, avoiding the NTP frequency skew altogether.
> 
> So remove KVM_REQ_GLOBAL_CLOCK_UPDATE. In kvm_write_system_time(), all
> that's needed is a single KVM_REQ_CLOCK_UPDATE for the vCPU whose KVM
> clock is being configured.
> 
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> ---
>   arch/x86/include/asm/kvm_host.h |  2 +-
>   arch/x86/kvm/x86.c              | 57 ++-------------------------------
>   2 files changed, 4 insertions(+), 55 deletions(-)
> 

Reviewed-by: Paul Durrant <paul@xen.org>
diff mbox series

Patch

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 8440c4081727..cfac72b4aa64 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -98,7 +98,7 @@ 
 	KVM_ARCH_REQ_FLAGS(14, KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP)
 #define KVM_REQ_SCAN_IOAPIC \
 	KVM_ARCH_REQ_FLAGS(15, KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP)
-#define KVM_REQ_GLOBAL_CLOCK_UPDATE	KVM_ARCH_REQ(16)
+/* KVM_ARCH_REQ(16) is available to be reused */
 #define KVM_REQ_APIC_PAGE_RELOAD \
 	KVM_ARCH_REQ_FLAGS(17, KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP)
 #define KVM_REQ_HV_CRASH		KVM_ARCH_REQ(18)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4ec4eb850c5b..f870e29d2558 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2354,13 +2354,13 @@  static void kvm_write_system_time(struct kvm_vcpu *vcpu, gpa_t system_time,
 	}
 
 	vcpu->arch.time = system_time;
-	kvm_make_request(KVM_REQ_GLOBAL_CLOCK_UPDATE, vcpu);
 
 	/* we verify if the enable bit is set... */
-	if (system_time & 1)
+	if (system_time & 1) {
 		kvm_gpc_activate(&vcpu->arch.pv_time, system_time & ~1ULL,
 				 sizeof(struct pvclock_vcpu_time_info));
-	else
+		kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
+	} else
 		kvm_gpc_deactivate(&vcpu->arch.pv_time);
 
 	return;
@@ -3391,46 +3391,6 @@  uint64_t kvm_get_wall_clock_epoch(struct kvm *kvm)
 	return ktime_get_real_ns() - get_kvmclock_ns(kvm);
 }
 
-/*
- * kvmclock updates which are isolated to a given vcpu, such as
- * vcpu->cpu migration, should not allow system_timestamp from
- * the rest of the vcpus to remain static. Otherwise ntp frequency
- * correction applies to one vcpu's system_timestamp but not
- * the others.
- *
- * So in those cases, request a kvmclock update for all vcpus.
- * We need to rate-limit these requests though, as they can
- * considerably slow guests that have a large number of vcpus.
- * The time for a remote vcpu to update its kvmclock is bound
- * by the delay we use to rate-limit the updates.
- */
-
-#define KVMCLOCK_UPDATE_DELAY msecs_to_jiffies(100)
-
-static void kvmclock_update_fn(struct work_struct *work)
-{
-	unsigned long i;
-	struct delayed_work *dwork = to_delayed_work(work);
-	struct kvm_arch *ka = container_of(dwork, struct kvm_arch,
-					   kvmclock_update_work);
-	struct kvm *kvm = container_of(ka, struct kvm, arch);
-	struct kvm_vcpu *vcpu;
-
-	kvm_for_each_vcpu(i, vcpu, kvm) {
-		kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
-		kvm_vcpu_kick(vcpu);
-	}
-}
-
-static void kvm_gen_kvmclock_update(struct kvm_vcpu *v)
-{
-	struct kvm *kvm = v->kvm;
-
-	kvm_make_request(KVM_REQ_CLOCK_UPDATE, v);
-	schedule_delayed_work(&kvm->arch.kvmclock_update_work,
-					KVMCLOCK_UPDATE_DELAY);
-}
-
 /* These helpers are safe iff @msr is known to be an MCx bank MSR. */
 static bool is_mci_control_msr(u32 msr)
 {
@@ -5018,12 +4978,6 @@  void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 		if (kvm_lapic_hv_timer_in_use(vcpu))
 			kvm_lapic_restart_hv_timer(vcpu);
 
-		/*
-		 * On a host with synchronized TSC, there is no need to update
-		 * kvmclock on vcpu->cpu migration
-		 */
-		if (!vcpu->kvm->arch.use_master_clock || vcpu->cpu == -1)
-			kvm_make_request(KVM_REQ_GLOBAL_CLOCK_UPDATE, vcpu);
 		if (vcpu->cpu != cpu)
 			kvm_make_request(KVM_REQ_MIGRATE_TIMER, vcpu);
 		vcpu->cpu = cpu;
@@ -10961,8 +10915,6 @@  static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 			__kvm_migrate_timers(vcpu);
 		if (kvm_check_request(KVM_REQ_MASTERCLOCK_UPDATE, vcpu))
 			kvm_update_masterclock(vcpu->kvm);
-		if (kvm_check_request(KVM_REQ_GLOBAL_CLOCK_UPDATE, vcpu))
-			kvm_gen_kvmclock_update(vcpu);
 		if (kvm_check_request(KVM_REQ_CLOCK_UPDATE, vcpu)) {
 			r = kvm_guest_time_update(vcpu);
 			if (unlikely(r))
@@ -12780,8 +12732,6 @@  int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
 	kvm->arch.hv_root_tdp = INVALID_PAGE;
 #endif
 
-	INIT_DELAYED_WORK(&kvm->arch.kvmclock_update_work, kvmclock_update_fn);
-
 	kvm_apicv_init(kvm);
 	kvm_hv_init_vm(kvm);
 	kvm_xen_init_vm(kvm);
@@ -12820,7 +12770,6 @@  static void kvm_unload_vcpu_mmus(struct kvm *kvm)
 
 void kvm_arch_sync_events(struct kvm *kvm)
 {
-	cancel_delayed_work_sync(&kvm->arch.kvmclock_update_work);
 	kvm_free_pit(kvm);
 }