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Message-ID: <CANRm+CyOQ+fDquCe2R4E9isSUiZTZR0Htr1JAz7jHr=xZ7GOYg@mail.gmail.com>
Date: Thu, 13 Jun 2019 09:50:49 +0800
From: Wanpeng Li <kernellwp@...il.com>
To: Sean Christopherson <sean.j.christopherson@...el.com>
Cc: LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
Paolo Bonzini <pbonzini@...hat.com>,
Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v3 1/2] KVM: LAPIC: Optimize timer latency consider world
switch time
On Wed, 12 Jun 2019 at 23:14, Sean Christopherson
<sean.j.christopherson@...el.com> wrote:
>
> On Wed, Jun 12, 2019 at 05:40:18PM +0800, Wanpeng Li wrote:
> > From: Wanpeng Li <wanpengli@...cent.com>
> >
> > Advance lapic timer tries to hidden the hypervisor overhead between the
> > host emulated timer fires and the guest awares the timer is fired. However,
> > even though after more sustaining optimizations, kvm-unit-tests/tscdeadline_latency
> > still awares ~1000 cycles latency since we lost the time between the end of
> > wait_lapic_expire and the guest awares the timer is fired. There are
> > codes between the end of wait_lapic_expire and the world switch, furthermore,
> > the world switch itself also has overhead. Actually the guest_tsc is equal
> > to the target deadline time in wait_lapic_expire is too late, guest will
> > aware the latency between the end of wait_lapic_expire() and after vmentry
> > to the guest. This patch takes this time into consideration.
> >
> > The vmentry+vmexit time which is measured by kvm-unit-tests/vmexit.falt is
> > 1800 cycles on my 2.5GHz Skylake server, the vmentry_advance_ns module
> > parameter default value is 300ns, it can be tuned/reworked in the further.
> > This patch can reduce average cyclictest latency from 3us to 2us on Skylake
> > server. (guest w/ nohz=off, idle=poll, host w/ preemption_timer=N, the
> > cyclictest latency is not too sensitive when preemption_timer=Y for this
> > optimization in my testing).
> >
> > Cc: Paolo Bonzini <pbonzini@...hat.com>
> > Cc: Radim Krčmář <rkrcmar@...hat.com>
> > Cc: Sean Christopherson <sean.j.christopherson@...el.com>
> > Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> > ---
> > v2 -> v3:
> > * read-only module parameter
> > * get_vmentry_advance_cycles() not inline
> > v1 -> v2:
> > * rename get_vmentry_advance_delta to get_vmentry_advance_cycles
> > * cache vmentry_advance_cycles by setting param bit 0
> > * add param max limit
> >
> > arch/x86/kvm/lapic.c | 33 ++++++++++++++++++++++++++++++---
> > arch/x86/kvm/lapic.h | 3 +++
> > arch/x86/kvm/vmx/vmx.c | 2 +-
> > arch/x86/kvm/x86.c | 8 ++++++++
> > arch/x86/kvm/x86.h | 2 ++
> > 5 files changed, 44 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > index fcf42a3..c6d76f9 100644
> > --- a/arch/x86/kvm/lapic.c
> > +++ b/arch/x86/kvm/lapic.c
> > @@ -1531,6 +1531,33 @@ static inline void adjust_lapic_timer_advance(struct kvm_vcpu *vcpu,
> > apic->lapic_timer.timer_advance_ns = timer_advance_ns;
> > }
> >
> > +u64 compute_vmentry_advance_cycles(struct kvm_vcpu *vcpu)
> > +{
> > + u64 cycles;
> > + struct kvm_lapic *apic = vcpu->arch.apic;
> > +
> > + cycles = vmentry_advance_ns * vcpu->arch.virtual_tsc_khz;
> > + do_div(cycles, 1000000);
> > +
> > + apic->lapic_timer.vmentry_advance_cycles = cycles;
> > +
> > + return cycles;
> > +}
> > +
> > +u64 get_vmentry_advance_cycles(struct kvm_vcpu *vcpu)
> > +{
> > + struct kvm_lapic *apic = vcpu->arch.apic;
> > +
> > + if (unlikely(!vmentry_advance_ns))
> > + return 0;
> > +
> > + if (likely(apic->lapic_timer.vmentry_advance_cycles))
> > + return apic->lapic_timer.vmentry_advance_cycles;
> > +
> > + return compute_vmentry_advance_cycles(vcpu);
>
> If vmentry_advance_ns is read-only, then we don't need to be able to
> compute lapic_timer.vmentry_advance_cycles on demand, e.g. it can be set
> during kvm_create_lapic() and recomputed in kvm_set_tsc_khz().
>
> Alternatively, it could be handled purely in kvm_set_tsc_khz() if the call
> to kvm_create_lapic() were moved before kvm_set_tsc_khz().
hardcode 25ns, how about something like this:
diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index fcf42a3..ed62d6b 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -1531,6 +1531,19 @@ static inline void
adjust_lapic_timer_advance(struct kvm_vcpu *vcpu,
apic->lapic_timer.timer_advance_ns = timer_advance_ns;
}
+u64 compute_vmentry_advance_cycles(struct kvm_vcpu *vcpu)
+{
+ u64 cycles;
+ struct kvm_lapic *apic = vcpu->arch.apic;
+
+ cycles = vmentry_advance_ns * vcpu->arch.virtual_tsc_khz;
+ do_div(cycles, 1000000);
+
+ apic->lapic_timer.vmentry_advance_cycles = cycles;
+
+ return cycles;
+}
+
void kvm_wait_lapic_expire(struct kvm_vcpu *vcpu)
{
struct kvm_lapic *apic = vcpu->arch.apic;
@@ -1544,7 +1557,8 @@ void kvm_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());
+ guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc()) +
+ apic->lapic_timer.vmentry_advance_cycles;
apic->lapic_timer.advance_expire_delta = guest_tsc - tsc_deadline;
if (guest_tsc < tsc_deadline)
@@ -1572,7 +1586,8 @@ static void start_sw_tscdeadline(struct kvm_lapic *apic)
local_irq_save(flags);
now = ktime_get();
- guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc());
+ guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc()) +
+ apic->lapic_timer.vmentry_advance_cycles;
ns = (tscdeadline - guest_tsc) * 1000000ULL;
do_div(ns, this_tsc_khz);
@@ -2329,7 +2344,7 @@ int kvm_create_lapic(struct kvm_vcpu *vcpu, int
timer_advance_ns)
apic->lapic_timer.timer_advance_ns = timer_advance_ns;
apic->lapic_timer.timer_advance_adjust_done = true;
}
-
+ apic->lapic_timer.vmentry_advance_cycles = 0;
/*
* APIC is created enabled. This will prevent kvm_lapic_set_base from
diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
index f974a3d..751cb26 100644
--- a/arch/x86/kvm/lapic.h
+++ b/arch/x86/kvm/lapic.h
@@ -33,6 +33,7 @@ struct kvm_timer {
u64 expired_tscdeadline;
u32 timer_advance_ns;
s64 advance_expire_delta;
+ u64 vmentry_advance_cycles;
atomic_t pending; /* accumulated triggered timers */
bool hv_timer_in_use;
bool timer_advance_adjust_done;
@@ -221,6 +222,7 @@ static inline int kvm_lapic_latched_init(struct
kvm_vcpu *vcpu)
bool kvm_apic_pending_eoi(struct kvm_vcpu *vcpu, int vector);
void kvm_wait_lapic_expire(struct kvm_vcpu *vcpu);
+u64 compute_vmentry_advance_cycles(struct kvm_vcpu *vcpu);
bool kvm_intr_is_single_vcpu_fast(struct kvm *kvm, struct kvm_lapic_irq *irq,
struct kvm_vcpu **dest_vcpu);
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 0861c71..b572359 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7041,7 +7041,8 @@ static int vmx_set_hv_timer(struct kvm_vcpu
*vcpu, u64 guest_deadline_tsc,
vmx = to_vmx(vcpu);
tscl = rdtsc();
- guest_tscl = kvm_read_l1_tsc(vcpu, tscl);
+ guest_tscl = kvm_read_l1_tsc(vcpu, tscl) +
+ vcpu->arch.apic->lapic_timer.vmentry_advance_cycles;
delta_tsc = max(guest_deadline_tsc, guest_tscl) - guest_tscl;
lapic_timer_advance_cycles = nsec_to_cycles(vcpu,
ktimer->timer_advance_ns);
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 553c292..f6e1366 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -145,6 +145,12 @@ module_param(tsc_tolerance_ppm, uint, S_IRUGO | S_IWUSR);
static int __read_mostly lapic_timer_advance_ns = -1;
module_param(lapic_timer_advance_ns, int, S_IRUGO | S_IWUSR);
+/*
+ * lapic timer vmentry advance (tscdeadline mode only) in nanoseconds.
+ */
+u32 __read_mostly vmentry_advance_ns = 25;
+module_param(vmentry_advance_ns, uint, S_IRUGO);
+
static bool __read_mostly vector_hashing = true;
module_param(vector_hashing, bool, S_IRUGO);
@@ -1592,6 +1598,8 @@ static int kvm_set_tsc_khz(struct kvm_vcpu
*vcpu, u32 user_tsc_khz)
kvm_get_time_scale(user_tsc_khz * 1000LL, NSEC_PER_SEC,
&vcpu->arch.virtual_tsc_shift,
&vcpu->arch.virtual_tsc_mult);
+ if (user_tsc_khz != vcpu->arch.virtual_tsc_khz)
+ compute_vmentry_advance_cycles(vcpu);
vcpu->arch.virtual_tsc_khz = user_tsc_khz;
/*
@@ -9127,8 +9135,6 @@ int kvm_arch_vcpu_init(struct kvm_vcpu *vcpu)
}
vcpu->arch.pio_data = page_address(page);
- kvm_set_tsc_khz(vcpu, max_tsc_khz);
-
r = kvm_mmu_create(vcpu);
if (r < 0)
goto fail_free_pio_data;
@@ -9141,6 +9147,8 @@ int kvm_arch_vcpu_init(struct kvm_vcpu *vcpu)
} else
static_key_slow_inc(&kvm_no_apic_vcpu);
+ kvm_set_tsc_khz(vcpu, max_tsc_khz);
+
vcpu->arch.mce_banks = kzalloc(KVM_MAX_MCE_BANKS * sizeof(u64) * 4,
GFP_KERNEL_ACCOUNT);
if (!vcpu->arch.mce_banks) {
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index a470ff0..2174355 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -294,6 +294,8 @@ extern u64 kvm_supported_xcr0(void);
extern unsigned int min_timer_period_us;
+extern unsigned int vmentry_advance_ns;
+
extern bool enable_vmware_backdoor;
extern struct static_key kvm_no_apic_vcpu;
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