[PATCHv1 dont apply] RFC: kvm eoi PV using shared memory

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I got lots of useful feedback from v0 so I thought
sending out a brain dump again would be a good idea.
This is mainly to show how I'm trying to address the
comments I got from the previous round. Flames/feedback
are wellcome!

Changes from v0:
- Tweaked setup MSRs a bit
- Keep ISR bit set. Before reading ISR, test EOI in guest memory and clear
- Check priority for nested interrupts, we can
  enable optimization if new one is high priority
- Disable optimization for any interrupt handled by ioapic
  (This is because ioapic handles notifiers and pic and it generally gets messy.
   It's possible that we can optimize some ioapic-handled
   edge interrupts - but is it worth it?)
- A less intrusive change for guest apic (0 overhead without kvm)

---

I took a stub at implementing PV EOI using shared memory.
This should reduce the number of exits an interrupt
causes as much as by half.

A partially complete draft for both host and guest parts
is below.

The idea is simple: there's a bit, per APIC, in guest memory,
that tells the guest that it does not need EOI.
We set it before injecting an interrupt and clear
before injecting a nested one. Guest tests it using
a test and clear operation - this is necessary
so that host can detect interrupt nesting -
and if set, it can skip the EOI MSR.

There's a new MSR to set the address of said register
in guest memory. Otherwise not much changed:
- Guest EOI is not required
- Register is tested ISR is automatically cleared before injection
 
qemu support is incomplete - mostly for feature negotiation.
need to add some trace points to enable profiling.
No testing was done beyond compiling the kernel.

Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>

diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
index b97596e..c9c70ea 100644
--- a/arch/x86/include/asm/bitops.h
+++ b/arch/x86/include/asm/bitops.h
@@ -26,11 +26,13 @@
 #if __GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 1)
 /* Technically wrong, but this avoids compilation errors on some gcc
    versions. */
-#define BITOP_ADDR(x) "=m" (*(volatile long *) (x))
+#define BITOP_ADDR_CONSTRAINT "=m"
 #else
-#define BITOP_ADDR(x) "+m" (*(volatile long *) (x))
+#define BITOP_ADDR_CONSTRAINT "+m"
 #endif
 
+#define BITOP_ADDR(x) BITOP_ADDR_CONSTRAINT (*(volatile long *) (x))
+
 #define ADDR				BITOP_ADDR(addr)
 
 /*
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index e216ba0..3d09ef1 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -481,6 +481,12 @@ struct kvm_vcpu_arch {
 		u64 length;
 		u64 status;
 	} osvw;
+
+	struct {
+		u64 msr_val;
+		struct gfn_to_hva_cache data;
+		bool pending;
+	} eoi;
 };
 
 struct kvm_lpage_info {
diff --git a/arch/x86/include/asm/kvm_para.h b/arch/x86/include/asm/kvm_para.h
index 734c376..164376a 100644
--- a/arch/x86/include/asm/kvm_para.h
+++ b/arch/x86/include/asm/kvm_para.h
@@ -22,6 +22,7 @@
 #define KVM_FEATURE_CLOCKSOURCE2        3
 #define KVM_FEATURE_ASYNC_PF		4
 #define KVM_FEATURE_STEAL_TIME		5
+#define KVM_FEATURE_EOI			6
 
 /* The last 8 bits are used to indicate how to interpret the flags field
  * in pvclock structure. If no bits are set, all flags are ignored.
@@ -37,6 +38,8 @@
 #define MSR_KVM_SYSTEM_TIME_NEW 0x4b564d01
 #define MSR_KVM_ASYNC_PF_EN 0x4b564d02
 #define MSR_KVM_STEAL_TIME  0x4b564d03
+#define MSR_KVM_EOI_EN      0x4b564d04
+#define MSR_KVM_EOI_DISABLED 0x0L
 
 struct kvm_steal_time {
 	__u64 steal;
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index b8ba6e4..450aae4 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -39,6 +39,7 @@
 #include <asm/desc.h>
 #include <asm/tlbflush.h>
 #include <asm/idle.h>
+#include <asm/apic.h>
 
 static int kvmapf = 1;
 
@@ -290,6 +291,33 @@ static void kvm_register_steal_time(void)
 		cpu, __pa(st));
 }
 
+/* TODO: needs to be early? aligned? */
+static DEFINE_EARLY_PER_CPU(u8, apic_eoi, 0);
+
+/* Our own copy of __test_and_clear_bit to make sure
+ * it is done with a single instruction */
+static inline int kvm_test_and_clear_bit(int nr, volatile u8* addr)
+{
+	int oldbit;
+
+	asm volatile("btr %2,%1\n\t"
+		     "sbb %0,%0"
+		     : "=r" (oldbit),
+		     BITOP_ADDR_CONSTRAINT (*(volatile u8 *) (addr))
+		     : "Ir" (nr));
+	return oldbit;
+}
+
+static void (*kvm_guest_native_apic_write)(u32 reg, u32 val);
+static void kvm_guest_apic_write(u32 reg, u32 val)
+{
+	if (reg == APIC_EOI &&
+	    kvm_test_and_clear_bit(0, &__get_cpu_var(apic_eoi)))
+			return;
+
+	kvm_guest_native_apic_write(reg, val);
+}
+
 void __cpuinit kvm_guest_cpu_init(void)
 {
 	if (!kvm_para_available())
@@ -307,11 +335,18 @@ void __cpuinit kvm_guest_cpu_init(void)
 		       smp_processor_id());
 	}
 
+	
+	if (kvm_para_has_feature(KVM_FEATURE_EOI)) {
+		kvm_guest_native_apic_write = apic->write;
+		apic->write = kvm_guest_apic_write;
+		wrmsrl(MSR_KVM_EOI_EN, __pa(&__get_cpu_var(apic_eoi)));
+	}
+
 	if (has_steal_clock)
 		kvm_register_steal_time();
 }
 
-static void kvm_pv_disable_apf(void *unused)
+static void kvm_pv_disable_apf(void)
 {
 	if (!__get_cpu_var(apf_reason).enabled)
 		return;
@@ -323,11 +358,18 @@ static void kvm_pv_disable_apf(void *unused)
 	       smp_processor_id());
 }
 
+static void kvm_pv_guest_cpu_reboot(void *unused)
+{
+	if (kvm_para_has_feature(KVM_FEATURE_EOI))
+		wrmsrl(MSR_KVM_EOI_EN, MSR_KVM_EOI_DISABLED);
+	kvm_pv_disable_apf();
+}
+
 static int kvm_pv_reboot_notify(struct notifier_block *nb,
 				unsigned long code, void *unused)
 {
 	if (code == SYS_RESTART)
-		on_each_cpu(kvm_pv_disable_apf, NULL, 1);
+		on_each_cpu(kvm_pv_guest_cpu_reboot, NULL, 1);
 	return NOTIFY_DONE;
 }
 
@@ -378,7 +420,9 @@ static void __cpuinit kvm_guest_cpu_online(void *dummy)
 static void kvm_guest_cpu_offline(void *dummy)
 {
 	kvm_disable_steal_time();
-	kvm_pv_disable_apf(NULL);
+	if (kvm_para_has_feature(KVM_FEATURE_EOI))
+		wrmsrl(MSR_KVM_EOI_EN, MSR_KVM_EOI_DISABLED);
+	kvm_pv_disable_apf();
 	apf_task_wake_all();
 }
 
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 9fed5be..7d00d2d 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -408,6 +408,7 @@ static int do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
 			     (1 << KVM_FEATURE_NOP_IO_DELAY) |
 			     (1 << KVM_FEATURE_CLOCKSOURCE2) |
 			     (1 << KVM_FEATURE_ASYNC_PF) |
+			     (1 << KVM_FEATURE_EOI) |
 			     (1 << KVM_FEATURE_CLOCKSOURCE_STABLE_BIT);
 
 		if (sched_info_on())
diff --git a/arch/x86/kvm/irq.c b/arch/x86/kvm/irq.c
index 7e06ba1..07bdfab 100644
--- a/arch/x86/kvm/irq.c
+++ b/arch/x86/kvm/irq.c
@@ -48,7 +48,7 @@ int kvm_cpu_has_interrupt(struct kvm_vcpu *v)
 	if (!irqchip_in_kernel(v->kvm))
 		return v->arch.interrupt.pending;
 
-	if (kvm_apic_has_interrupt(v) == -1) {	/* LAPIC */
+	if (kvm_apic_has_interrupt(v) < 0) {	/* LAPIC */
 		if (kvm_apic_accept_pic_intr(v)) {
 			s = pic_irqchip(v->kvm);	/* PIC */
 			return s->output;
diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 992b4ea..c2118ab 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -270,6 +270,54 @@ int kvm_apic_set_irq(struct kvm_vcpu *vcpu, struct kvm_lapic_irq *irq)
 			irq->level, irq->trig_mode);
 }
 
+static int eoi_put_user(struct kvm_vcpu *vcpu, u8 val)
+{
+
+	return kvm_write_guest_cached(vcpu->kvm, &vcpu->arch.eoi.data, &val,
+				      sizeof(val));
+}
+
+static int eoi_get_user(struct kvm_vcpu *vcpu, u8 *val)
+{
+
+	return kvm_read_guest_cached(vcpu->kvm, &vcpu->arch.eoi.data, val,
+				      sizeof(*val));
+}
+
+static inline bool eoi_enabled(struct kvm_vcpu *vcpu)
+{
+	return vcpu->arch.eoi.msr_val != MSR_KVM_EOI_DISABLED;
+}
+
+static bool eoi_get_pending(struct kvm_vcpu *vcpu)
+{
+	u8 val;
+	if (eoi_get_user(vcpu, &val) < 0)
+		apic_debug("Can't read EOI MSR value: 0x%llx\n",
+			   (unsigned long long)vcpi->arch.eoi.msr_val);
+	return val & 0x1;
+}
+
+static void eoi_set_pending(struct kvm_vcpu *vcpu)
+{
+	if (eoi_put_user(vcpu, 0x1) < 0) {
+		apic_debug("Can't set EOI MSR value: 0x%llx\n",
+			   (unsigned long long)vcpi->arch.eoi.msr_val);
+		return;
+	}
+	vcpu->arch.eoi.pending = true;
+}
+
+static void eoi_clr_pending(struct kvm_vcpu *vcpu)
+{
+	if (eoi_put_user(vcpu, 0x0) < 0) {
+		apic_debug("Can't clear EOI MSR value: 0x%llx\n",
+			   (unsigned long long)vcpi->arch.eoi.msr_val);
+		return;
+	}
+	vcpu->arch.eoi.pending = false;
+}
+
 static inline int apic_find_highest_isr(struct kvm_lapic *apic)
 {
 	int result;
@@ -280,6 +328,20 @@ static inline int apic_find_highest_isr(struct kvm_lapic *apic)
 	return result;
 }
 
+static void apic_update_isr(struct kvm_lapic *apic)
+{
+	int vector;
+	if (!eoi_enabled(apic->vcpu) ||
+	    !apic->vcpu->arch.eoi.pending ||
+	    eoi_get_pending(apic->vcpu))
+		return;
+	apic->vcpu->arch.eoi.pending = false;
+	vector = apic_find_highest_isr(apic);
+	if (vector == -1)
+		return;
+	apic_clear_vector(vector, apic->regs + APIC_ISR);
+}
+
 static void apic_update_ppr(struct kvm_lapic *apic)
 {
 	u32 tpr, isrv, ppr, old_ppr;
@@ -287,6 +349,7 @@ static void apic_update_ppr(struct kvm_lapic *apic)
 
 	old_ppr = apic_get_reg(apic, APIC_PROCPRI);
 	tpr = apic_get_reg(apic, APIC_TASKPRI);
+	apic_update_isr(apic);
 	isr = apic_find_highest_isr(apic);
 	isrv = (isr != -1) ? isr : 0;
 
@@ -492,6 +555,8 @@ static void apic_set_eoi(struct kvm_lapic *apic)
 	if (vector == -1)
 		return;
 
+	if (eoi_enabled(apic->vcpu))
+		eoi_clr_pending(apic->vcpu);
 	apic_clear_vector(vector, apic->regs + APIC_ISR);
 	apic_update_ppr(apic);
 
@@ -1085,6 +1150,8 @@ void kvm_lapic_reset(struct kvm_vcpu *vcpu)
 	atomic_set(&apic->lapic_timer.pending, 0);
 	if (kvm_vcpu_is_bsp(vcpu))
 		vcpu->arch.apic_base |= MSR_IA32_APICBASE_BSP;
+	vcpu->arch.eoi.msr_val = MSR_KVM_EOI_DISABLED;
+	vcpu->arch.eoi.pending = false;
 	apic_update_ppr(apic);
 
 	vcpu->arch.apic_arb_prio = 0;
@@ -1210,9 +1277,10 @@ int kvm_apic_has_interrupt(struct kvm_vcpu *vcpu)
 
 	apic_update_ppr(apic);
 	highest_irr = apic_find_highest_irr(apic);
-	if ((highest_irr == -1) ||
-	    ((highest_irr & 0xF0) <= apic_get_reg(apic, APIC_PROCPRI)))
+	if (highest_irr == -1)
 		return -1;
+	if (((highest_irr & 0xF0) <= apic_get_reg(apic, APIC_PROCPRI)))
+		return -2;
 	return highest_irr;
 }
 
@@ -1244,9 +1312,20 @@ int kvm_get_apic_interrupt(struct kvm_vcpu *vcpu)
 	int vector = kvm_apic_has_interrupt(vcpu);
 	struct kvm_lapic *apic = vcpu->arch.apic;
 
-	if (vector == -1)
+	/* Detect interrupt nesting and disable EOI optimization */
+	if (eoi_enabled(vcpu) && vector == -2)
+		eoi_clr_pending(vcpu);
+
+	if (vector < 0)
 		return -1;
 
+	if (eoi_enabled(vcpu)) {
+		if (kvm_ioapic_handles_vector(vcpu->kvm, vector))
+			eoi_clr_pending(vcpu);
+		else
+			eoi_set_pending(vcpu);
+	}
+
 	apic_set_vector(vector, apic->regs + APIC_ISR);
 	apic_update_ppr(apic);
 	apic_clear_irr(vector, apic);
@@ -1261,6 +1340,8 @@ void kvm_apic_post_state_restore(struct kvm_vcpu *vcpu)
 			     MSR_IA32_APICBASE_BASE;
 	kvm_apic_set_version(vcpu);
 
+	if (eoi_enabled(apic->vcpu))
+	    apic->vcpu->arch.eoi.pending = eoi_get_pending(apic->vcpu);
 	apic_update_ppr(apic);
 	hrtimer_cancel(&apic->lapic_timer.timer);
 	update_divide_count(apic);
@@ -1392,3 +1473,18 @@ int kvm_hv_vapic_msr_read(struct kvm_vcpu *vcpu, u32 reg, u64 *data)
 
 	return 0;
 }
+
+int kvm_pv_enable_apic_eoi(struct kvm_vcpu *vcpu, u64 data)
+{
+	if (data == MSR_KVM_EOI_DISABLED) {
+		struct kvm_lapic *apic = vcpu->arch.apic;
+		if (apic && apic_enabled(apic))
+			apic_update_isr(apic);
+	} else if (kvm_gfn_to_hva_cache_init(vcpu->kvm, &vcpu->arch.eoi.data,
+					     data))
+		return 1;
+
+	vcpu->arch.eoi.msr_val = data;
+	vcpu->arch.eoi.pending = false;
+	return 0;
+}
diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
index 6f4ce25..042dace 100644
--- a/arch/x86/kvm/lapic.h
+++ b/arch/x86/kvm/lapic.h
@@ -60,4 +60,6 @@ static inline bool kvm_hv_vapic_assist_page_enabled(struct kvm_vcpu *vcpu)
 {
 	return vcpu->arch.hv_vapic & HV_X64_MSR_APIC_ASSIST_PAGE_ENABLE;
 }
+
+int kvm_pv_enable_apic_eoi(struct kvm_vcpu *vcpu, u64 data);
 #endif
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4044ce0..31d6762 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1627,6 +1627,10 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, u32 msr, u64 data)
 		if (kvm_pv_enable_async_pf(vcpu, data))
 			return 1;
 		break;
+	case MSR_KVM_EOI_EN:
+		if (kvm_pv_enable_apic_eoi(vcpu, data))
+			return 1;
+		break;
 	case MSR_KVM_STEAL_TIME:
 
 		if (unlikely(!sched_info_on()))
--
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