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Subject[PATCH v2 3/4] KVM: LAPIC: Expose per-vCPU timer adavance information to userspace
Date
From: Wanpeng Li <wanpengli@tencent.com>

Expose the per-vCPU advancement information to the user via per-vCPU debugfs
entry. wait_lapic_expire() call was moved above guest_enter_irqoff() because
of its tracepoint, which violated the RCU extended quiescent state invoked
by guest_enter_irqoff()[1][2]. This patch simply removes the tracepoint,
which would allow moving wait_lapic_expire(). Sean pointed out:

| Now that the advancement time is tracked per-vCPU, realizing a change
| in the advancement time requires creating a new VM. For all intents
| and purposes this makes it impractical to hand tune the advancement
| in real time using the tracepoint as the feedback mechanism.

[1] Commit 8b89fe1f6c43 ("kvm: x86: move tracepoints outside extended quiescent state")
[2] https://patchwork.kernel.org/patch/7821111/

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Liran Alon <liran.alon@oracle.com>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
---
arch/x86/kvm/debugfs.c | 16 ++++++++++++++++
arch/x86/kvm/lapic.c | 16 ++++++++--------
arch/x86/kvm/lapic.h | 1 +
arch/x86/kvm/trace.h | 20 --------------------
4 files changed, 25 insertions(+), 28 deletions(-)

diff --git a/arch/x86/kvm/debugfs.c b/arch/x86/kvm/debugfs.c
index c19c7ed..8cf542e 100644
--- a/arch/x86/kvm/debugfs.c
+++ b/arch/x86/kvm/debugfs.c
@@ -9,12 +9,22 @@
*/
#include <linux/kvm_host.h>
#include <linux/debugfs.h>
+#include "lapic.h"

bool kvm_arch_has_vcpu_debugfs(void)
{
return true;
}

+static int vcpu_get_timer_expire_delta(void *data, u64 *val)
+{
+ struct kvm_vcpu *vcpu = (struct kvm_vcpu *) data;
+ *val = vcpu->arch.apic->lapic_timer.advance_expire_delta;
+ return 0;
+}
+
+DEFINE_SIMPLE_ATTRIBUTE(vcpu_timer_expire_delta_fops, vcpu_get_timer_expire_delta, NULL, "%lld\n");
+
static int vcpu_get_tsc_offset(void *data, u64 *val)
{
struct kvm_vcpu *vcpu = (struct kvm_vcpu *) data;
@@ -51,6 +61,12 @@ int kvm_arch_create_vcpu_debugfs(struct kvm_vcpu *vcpu)
if (!ret)
return -ENOMEM;

+ ret = debugfs_create_file("advance_expire_delta", 0444,
+ vcpu->debugfs_dentry,
+ vcpu, &vcpu_timer_expire_delta_fops);
+ if (!ret)
+ return -ENOMEM;
+
if (kvm_has_tsc_control) {
ret = debugfs_create_file("tsc-scaling-ratio", 0444,
vcpu->debugfs_dentry,
diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 2f364fe..af38ece 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -1502,27 +1502,27 @@ static inline void __wait_lapic_expire(struct kvm_vcpu *vcpu, u64 guest_cycles)
}

static inline void adaptive_tune_timer_advancement(struct kvm_vcpu *vcpu,
- u64 guest_tsc, u64 tsc_deadline)
+ s64 advance_expire_delta)
{
struct kvm_lapic *apic = vcpu->arch.apic;
u32 timer_advance_ns = apic->lapic_timer.timer_advance_ns;
u64 ns;

/* too early */
- if (guest_tsc < tsc_deadline) {
- ns = (tsc_deadline - guest_tsc) * 1000000ULL;
+ if (advance_expire_delta < 0) {
+ ns = -advance_expire_delta * 1000000ULL;
do_div(ns, vcpu->arch.virtual_tsc_khz);
timer_advance_ns -= min((u32)ns,
timer_advance_ns / LAPIC_TIMER_ADVANCE_ADJUST_STEP);
} else {
/* too late */
- ns = (guest_tsc - tsc_deadline) * 1000000ULL;
+ ns = advance_expire_delta * 1000000ULL;
do_div(ns, vcpu->arch.virtual_tsc_khz);
timer_advance_ns += min((u32)ns,
timer_advance_ns / LAPIC_TIMER_ADVANCE_ADJUST_STEP);
}

- if (abs(guest_tsc - tsc_deadline) < LAPIC_TIMER_ADVANCE_ADJUST_DONE)
+ if (abs(advance_expire_delta) < LAPIC_TIMER_ADVANCE_ADJUST_DONE)
apic->lapic_timer.timer_advance_adjust_done = true;
if (unlikely(timer_advance_ns > 5000)) {
timer_advance_ns = 0;
@@ -1545,13 +1545,13 @@ void wait_lapic_expire(struct kvm_vcpu *vcpu)
tsc_deadline = apic->lapic_timer.expired_tscdeadline;
apic->lapic_timer.expired_tscdeadline = 0;
guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc());
- trace_kvm_wait_lapic_expire(vcpu->vcpu_id, guest_tsc - tsc_deadline);
+ apic->lapic_timer.advance_expire_delta = guest_tsc - tsc_deadline;

- if (guest_tsc < tsc_deadline)
+ if (apic->lapic_timer.advance_expire_delta < 0)
__wait_lapic_expire(vcpu, tsc_deadline - guest_tsc);

if (unlikely(!apic->lapic_timer.timer_advance_adjust_done))
- adaptive_tune_timer_advancement(vcpu, guest_tsc, tsc_deadline);
+ adaptive_tune_timer_advancement(vcpu, apic->lapic_timer.advance_expire_delta);
}

static void start_sw_tscdeadline(struct kvm_lapic *apic)
diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
index d6d049b..3e72a25 100644
--- a/arch/x86/kvm/lapic.h
+++ b/arch/x86/kvm/lapic.h
@@ -32,6 +32,7 @@ struct kvm_timer {
u64 tscdeadline;
u64 expired_tscdeadline;
u32 timer_advance_ns;
+ s64 advance_expire_delta;
atomic_t pending; /* accumulated triggered timers */
bool hv_timer_in_use;
bool timer_advance_adjust_done;
diff --git a/arch/x86/kvm/trace.h b/arch/x86/kvm/trace.h
index 4d47a26..3f9bc62 100644
--- a/arch/x86/kvm/trace.h
+++ b/arch/x86/kvm/trace.h
@@ -953,26 +953,6 @@ TRACE_EVENT(kvm_pvclock_update,
__entry->flags)
);

-TRACE_EVENT(kvm_wait_lapic_expire,
- TP_PROTO(unsigned int vcpu_id, s64 delta),
- TP_ARGS(vcpu_id, delta),
-
- TP_STRUCT__entry(
- __field( unsigned int, vcpu_id )
- __field( s64, delta )
- ),
-
- TP_fast_assign(
- __entry->vcpu_id = vcpu_id;
- __entry->delta = delta;
- ),
-
- TP_printk("vcpu %u: delta %lld (%s)",
- __entry->vcpu_id,
- __entry->delta,
- __entry->delta < 0 ? "early" : "late")
-);
-
TRACE_EVENT(kvm_enter_smm,
TP_PROTO(unsigned int vcpu_id, u64 smbase, bool entering),
TP_ARGS(vcpu_id, smbase, entering),
--
2.7.4
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