[PATCH 5/8] KVM: x86: acknowledgment mechanism for async pf page ready notifications

Vitaly Kuznetsov
2020-05-11T16:48:47Z

If two page ready notifications happen back to back the second one is not
delivered and the only mechanism we currently have is
kvm_check_async_pf_completion() check in vcpu_run() loop. The check will
only be performed with the next vmexit when it happens and in some cases
it may take a while. With interrupt based page ready notification delivery
the situation is even worse: unlike exceptions, interrupts are not handled
immediately so we must check if the slot is empty. This is slow and
unnecessary. Introduce dedicated MSR_KVM_ASYNC_PF_ACK MSR to communicate
the fact that the slot is free and host should check its notification
queue. Mandate using it for interrupt based type 2 APF event delivery.

As kvm_check_async_pf_completion() is going away from vcpu_run() we need
a way to communicate the fact that vcpu->async_pf.done queue has
transitioned from empty to non-empty state. Introduce
kvm_arch_async_page_present_queued() and KVM_REQ_APF_READY to do the job.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
Documentation/virt/kvm/msr.rst | 18 +++++++++++++++---
arch/s390/include/asm/kvm_host.h | 2 ++
arch/x86/include/asm/kvm_host.h | 3 +++
arch/x86/include/uapi/asm/kvm_para.h | 1 +
arch/x86/kvm/x86.c | 26 ++++++++++++++++++++++----
virt/kvm/async_pf.c | 10 ++++++++++
6 files changed, 53 insertions(+), 7 deletions(-)

diff --git a/Documentation/virt/kvm/msr.rst b/Documentation/virt/kvm/msr.rst
index f988a36f226a..3c7afbcd243f 100644
--- a/Documentation/virt/kvm/msr.rst
+++ b/Documentation/virt/kvm/msr.rst
@@ -230,9 +230,10 @@ data:
The content of these bytes is a token which was previously delivered as
type 1 event. The event indicates the page in now available. Guest is
supposed to write '0' to the location when it is done handling type 2
- event so the next one can be delivered. MSR_KVM_ASYNC_PF_INT MSR
- specifying the interrupt vector for type 2 APF delivery needs to be
- written to before enabling APF mechanism in MSR_KVM_ASYNC_PF_EN.
+ event so the next one can be delivered. It is also supposed to write
+ '1' to MSR_KVM_ASYNC_PF_ACK every time after clearing the location,
+ this forces KVM to re-scan its queue and deliver next pending
+ notification.

Note, previously, type 2 (page present) events were delivered via the
same #PF exception as type 1 (page not present) events but this is
@@ -352,3 +353,14 @@ data:
Interrupt vector for asynchnonous page ready notifications delivery.
The vector has to be set up before asynchronous page fault mechanism
is enabled in MSR_KVM_ASYNC_PF_EN.
+
+MSR_KVM_ASYNC_PF_ACK:
+ 0x4b564d07
+
+data:
+ Asynchronous page fault (APF) acknowledgment.
+
+ When the guest is done processing type 2 APF event and 'pageready_token'
+ field in 'struct kvm_vcpu_pv_apf_data' is cleared it is supposed to
+ write '1' to bit 0 of the MSR, this caused the host to re-scan its queue
+ and check if there are more notifications pending.
diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h
index d6bcd34f3ec3..c9c5b8b71175 100644
--- a/arch/s390/include/asm/kvm_host.h
+++ b/arch/s390/include/asm/kvm_host.h
@@ -982,6 +982,8 @@ void kvm_arch_async_page_not_present(struct kvm_vcpu *vcpu,
void kvm_arch_async_page_present(struct kvm_vcpu *vcpu,
struct kvm_async_pf *work);

+static inline void kvm_arch_async_page_present_queued(struct kvm_vcpu *vcpu) {}
+
void kvm_arch_crypto_clear_masks(struct kvm *kvm);
void kvm_arch_crypto_set_masks(struct kvm *kvm, unsigned long *apm,
unsigned long *aqm, unsigned long *adm);
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 4a72ba26a532..0eeeb98ed02a 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -83,6 +83,7 @@
#define KVM_REQ_GET_VMCS12_PAGES KVM_ARCH_REQ(24)
#define KVM_REQ_APICV_UPDATE \
KVM_ARCH_REQ_FLAGS(25, KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP)
+#define KVM_REQ_APF_READY KVM_ARCH_REQ(26)

#define CR0_RESERVED_BITS \
(~(unsigned long)(X86_CR0_PE | X86_CR0_MP | X86_CR0_EM | X86_CR0_TS \
@@ -771,6 +772,7 @@ struct kvm_vcpu_arch {
u32 host_apf_reason;
unsigned long nested_apf_token;
bool delivery_as_pf_vmexit;
+ bool pageready_pending;
} apf;

/* OSVW MSRs (AMD only) */
@@ -1643,6 +1645,7 @@ void kvm_arch_async_page_present(struct kvm_vcpu *vcpu,
struct kvm_async_pf *work);
void kvm_arch_async_page_ready(struct kvm_vcpu *vcpu,
struct kvm_async_pf *work);
+void kvm_arch_async_page_present_queued(struct kvm_vcpu *vcpu);
bool kvm_arch_can_inject_async_page_present(struct kvm_vcpu *vcpu);
extern bool kvm_find_async_pf_gfn(struct kvm_vcpu *vcpu, gfn_t gfn);

diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
index c55ffa2c40b6..941fdf4569a4 100644
--- a/arch/x86/include/uapi/asm/kvm_para.h
+++ b/arch/x86/include/uapi/asm/kvm_para.h
@@ -51,6 +51,7 @@
#define MSR_KVM_PV_EOI_EN 0x4b564d04
#define MSR_KVM_POLL_CONTROL 0x4b564d05
#define MSR_KVM_ASYNC_PF_INT 0x4b564d06
+#define MSR_KVM_ASYNC_PF_ACK 0x4b564d07

struct kvm_steal_time {
__u64 steal;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index d1e9b072d279..d21adc23a99f 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1243,7 +1243,7 @@ static const u32 emulated_msrs_all[] = {
HV_X64_MSR_TSC_EMULATION_STATUS,

MSR_KVM_ASYNC_PF_EN, MSR_KVM_STEAL_TIME,
- MSR_KVM_PV_EOI_EN, MSR_KVM_ASYNC_PF_INT,
+ MSR_KVM_PV_EOI_EN, MSR_KVM_ASYNC_PF_INT, MSR_KVM_ASYNC_PF_ACK,

MSR_IA32_TSC_ADJUST,
MSR_IA32_TSCDEADLINE,
@@ -2912,6 +2912,12 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
if (kvm_pv_enable_async_pf_int(vcpu, data))
return 1;
break;
+ case MSR_KVM_ASYNC_PF_ACK:
+ if (data & 0x1) {
+ vcpu->arch.apf.pageready_pending = false;
+ kvm_check_async_pf_completion(vcpu);
+ }
+ break;
case MSR_KVM_STEAL_TIME:

if (unlikely(!sched_info_on()))
@@ -3191,6 +3197,9 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
case MSR_KVM_ASYNC_PF_INT:
msr_info->data = vcpu->arch.apf.msr_int_val;
break;
+ case MSR_KVM_ASYNC_PF_ACK:
+ msr_info->data = 0;
+ break;
case MSR_KVM_STEAL_TIME:
msr_info->data = vcpu->arch.st.msr_val;
break;
@@ -8336,6 +8345,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
kvm_hv_process_stimers(vcpu);
if (kvm_check_request(KVM_REQ_APICV_UPDATE, vcpu))
kvm_vcpu_update_apicv(vcpu);
+ if (kvm_check_request(KVM_REQ_APF_READY, vcpu))
+ kvm_check_async_pf_completion(vcpu);
}

if (kvm_check_request(KVM_REQ_EVENT, vcpu) || req_int_win) {
@@ -8610,8 +8621,6 @@ static int vcpu_run(struct kvm_vcpu *vcpu)
break;
}

- kvm_check_async_pf_completion(vcpu);
-
if (signal_pending(current)) {
r = -EINTR;
vcpu->run->exit_reason = KVM_EXIT_INTR;
@@ -10489,13 +10498,22 @@ void kvm_arch_async_page_present(struct kvm_vcpu *vcpu,
trace_kvm_async_pf_ready(work->arch.token, work->cr2_or_gpa);

if (kvm_pv_async_pf_enabled(vcpu) &&
- !apf_put_user_ready(vcpu, work->arch.token))
+ !apf_put_user_ready(vcpu, work->arch.token)) {
+ vcpu->arch.apf.pageready_pending = true;
kvm_apic_set_irq(vcpu, &irq, NULL);
+ }

vcpu->arch.apf.halted = false;
vcpu->arch.mp_state = KVM_MP_STATE_RUNNABLE;
}

+void kvm_arch_async_page_present_queued(struct kvm_vcpu *vcpu)
+{
+ kvm_make_request(KVM_REQ_APF_READY, vcpu);
+ if (!vcpu->arch.apf.pageready_pending)
+ kvm_vcpu_kick(vcpu);
+}
+
bool kvm_arch_can_inject_async_page_present(struct kvm_vcpu *vcpu)
{
if (!kvm_pv_async_pf_enabled(vcpu))
diff --git a/virt/kvm/async_pf.c b/virt/kvm/async_pf.c
index 15e5b037f92d..746d95cf98ad 100644
--- a/virt/kvm/async_pf.c
+++ b/virt/kvm/async_pf.c
@@ -51,6 +51,7 @@ static void async_pf_execute(struct work_struct *work)
unsigned long addr = apf->addr;
gpa_t cr2_or_gpa = apf->cr2_or_gpa;
int locked = 1;
+ bool first;

might_sleep();

@@ -69,10 +70,14 @@ static void async_pf_execute(struct work_struct *work)
kvm_arch_async_page_present(vcpu, apf);

spin_lock(&vcpu->async_pf.lock);
+ first = list_empty(&vcpu->async_pf.done);
list_add_tail(&apf->link, &vcpu->async_pf.done);
apf->vcpu = NULL;
spin_unlock(&vcpu->async_pf.lock);

+ if (!IS_ENABLED(CONFIG_KVM_ASYNC_PF_SYNC) && first)
+ kvm_arch_async_page_present_queued(vcpu);
+
/*
* apf may be freed by kvm_check_async_pf_completion() after
* this point
@@ -202,6 +207,7 @@ int kvm_setup_async_pf(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
int kvm_async_pf_wakeup_all(struct kvm_vcpu *vcpu)
{
struct kvm_async_pf *work;
+ bool first;

if (!list_empty_careful(&vcpu->async_pf.done))
return 0;
@@ -214,9 +220,13 @@ int kvm_async_pf_wakeup_all(struct kvm_vcpu *vcpu)
INIT_LIST_HEAD(&work->queue); /* for list_del to work */

spin_lock(&vcpu->async_pf.lock);
+ first = list_empty(&vcpu->async_pf.done);
list_add_tail(&work->link, &vcpu->async_pf.done);
spin_unlock(&vcpu->async_pf.lock);

+ if (!IS_ENABLED(CONFIG_KVM_ASYNC_PF_SYNC) && first)
+ kvm_arch_async_page_present_queued(vcpu);
+
vcpu->async_pf.queued++;
return 0;
}
--
2.25.4