[PATCHv6 3/8] kvm_para: guest side for eoi avoidance

Michael S. Tsirkin
2012-06-03T07:28:07Z

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

I run a simple microbenchmark to show exit reduction
(note: for testing, need to apply follow-up patch
'kvm: host side for eoi optimization' + a qemu patch
I posted separately, on host):

Before:

Performance counter stats for 'sleep 1s':

47,357 kvm:kvm_entry [99.98%]
0 kvm:kvm_hypercall [99.98%]
0 kvm:kvm_hv_hypercall [99.98%]
5,001 kvm:kvm_pio [99.98%]
0 kvm:kvm_cpuid [99.98%]
22,124 kvm:kvm_apic [99.98%]
49,849 kvm:kvm_exit [99.98%]
21,115 kvm:kvm_inj_virq [99.98%]
0 kvm:kvm_inj_exception [99.98%]
0 kvm:kvm_page_fault [99.98%]
22,937 kvm:kvm_msr [99.98%]
0 kvm:kvm_cr [99.98%]
0 kvm:kvm_pic_set_irq [99.98%]
0 kvm:kvm_apic_ipi [99.98%]
22,207 kvm:kvm_apic_accept_irq [99.98%]
22,421 kvm:kvm_eoi [99.98%]
0 kvm:kvm_pv_eoi [99.99%]
0 kvm:kvm_nested_vmrun [99.99%]
0 kvm:kvm_nested_intercepts [99.99%]
0 kvm:kvm_nested_vmexit [99.99%]
0 kvm:kvm_nested_vmexit_inject [99.99%]
0 kvm:kvm_nested_intr_vmexit [99.99%]
0 kvm:kvm_invlpga [99.99%]
0 kvm:kvm_skinit [99.99%]
57 kvm:kvm_emulate_insn [99.99%]
0 kvm:vcpu_match_mmio [99.99%]
0 kvm:kvm_userspace_exit [99.99%]
2 kvm:kvm_set_irq [99.99%]
2 kvm:kvm_ioapic_set_irq [99.99%]
23,609 kvm:kvm_msi_set_irq [99.99%]
1 kvm:kvm_ack_irq [99.99%]
131 kvm:kvm_mmio [99.99%]
226 kvm:kvm_fpu [100.00%]
0 kvm:kvm_age_page [100.00%]
0 kvm:kvm_try_async_get_page [100.00%]
0 kvm:kvm_async_pf_doublefault [100.00%]
0 kvm:kvm_async_pf_not_present [100.00%]
0 kvm:kvm_async_pf_ready [100.00%]
0 kvm:kvm_async_pf_completed

1.002100578 seconds time elapsed

After:

Performance counter stats for 'sleep 1s':

28,354 kvm:kvm_entry [99.98%]
0 kvm:kvm_hypercall [99.98%]
0 kvm:kvm_hv_hypercall [99.98%]
1,347 kvm:kvm_pio [99.98%]
0 kvm:kvm_cpuid [99.98%]
1,931 kvm:kvm_apic [99.98%]
29,595 kvm:kvm_exit [99.98%]
24,884 kvm:kvm_inj_virq [99.98%]
0 kvm:kvm_inj_exception [99.98%]
0 kvm:kvm_page_fault [99.98%]
1,986 kvm:kvm_msr [99.98%]
0 kvm:kvm_cr [99.98%]
0 kvm:kvm_pic_set_irq [99.98%]
0 kvm:kvm_apic_ipi [99.99%]
25,953 kvm:kvm_apic_accept_irq [99.99%]
26,132 kvm:kvm_eoi [99.99%]
26,593 kvm:kvm_pv_eoi [99.99%]
0 kvm:kvm_nested_vmrun [99.99%]
0 kvm:kvm_nested_intercepts [99.99%]
0 kvm:kvm_nested_vmexit [99.99%]
0 kvm:kvm_nested_vmexit_inject [99.99%]
0 kvm:kvm_nested_intr_vmexit [99.99%]
0 kvm:kvm_invlpga [99.99%]
0 kvm:kvm_skinit [99.99%]
284 kvm:kvm_emulate_insn [99.99%]
68 kvm:vcpu_match_mmio [99.99%]
68 kvm:kvm_userspace_exit [99.99%]
2 kvm:kvm_set_irq [99.99%]
2 kvm:kvm_ioapic_set_irq [99.99%]
28,288 kvm:kvm_msi_set_irq [99.99%]
1 kvm:kvm_ack_irq [99.99%]
131 kvm:kvm_mmio [100.00%]
588 kvm:kvm_fpu [100.00%]
0 kvm:kvm_age_page [100.00%]
0 kvm:kvm_try_async_get_page [100.00%]
0 kvm:kvm_async_pf_doublefault [100.00%]
0 kvm:kvm_async_pf_not_present [100.00%]
0 kvm:kvm_async_pf_ready [100.00%]
0 kvm:kvm_async_pf_completed

1.002039622 seconds time elapsed

We see that # of exits is almost halved.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
arch/x86/include/asm/bitops.h | 6 +++-
arch/x86/include/asm/kvm_para.h | 7 +++++
arch/x86/kernel/kvm.c | 51 ++++++++++++++++++++++++++++++++++++--
3 files changed, 59 insertions(+), 5 deletions(-)

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_para.h b/arch/x86/include/asm/kvm_para.h
index 63ab166..2f7712e 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_PV_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 +38,7 @@
#define MSR_KVM_SYSTEM_TIME_NEW 0x4b564d01
#define MSR_KVM_ASYNC_PF_EN 0x4b564d02
#define MSR_KVM_STEAL_TIME 0x4b564d03
+#define MSR_KVM_PV_EOI_EN 0x4b564d04

struct kvm_steal_time {
__u64 steal;
@@ -89,6 +91,11 @@ struct kvm_vcpu_pv_apf_data {
__u32 enabled;
};

+#define KVM_PV_EOI_BIT 0
+#define KVM_PV_EOI_MASK (0x1 << KVM_PV_EOI_BIT)
+#define KVM_PV_EOI_ENABLED KVM_PV_EOI_MASK
+#define KVM_PV_EOI_DISABLED 0x0
+
#ifdef __KERNEL__
#include <asm/processor.h>

diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index e554e5a..85cd6ac 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -39,6 +39,8 @@
#include <asm/desc.h>
#include <asm/tlbflush.h>
#include <asm/idle.h>
+#include <asm/apic.h>
+#include <asm/apicdef.h>

static int kvmapf = 1;

@@ -283,6 +285,24 @@ static void kvm_register_steal_time(void)
cpu, __pa(st));
}

+/* size alignment is implied but just to make it explicit. */
+static DEFINE_PER_CPU(unsigned long, kvm_apic_eoi) __aligned(2) =
+ KVM_PV_EOI_DISABLED;
+
+static void kvm_guest_apic_eoi_write(u32 reg, u32 val)
+{
+ /**
+ * This relies on __test_and_clear_bit to modify the memory
+ * in a way that is atomic with respect to the local CPU.
+ * The hypervisor only accesses this memory from the local CPU so
+ * there's no need for lock or memory barriers.
+ * An optimization barrier is implied in apic write.
+ */
+ if (__test_and_clear_bit(KVM_PV_EOI_BIT, &__get_cpu_var(kvm_apic_eoi)))
+ return;
+ apic->write(APIC_EOI, APIC_EOI_ACK);
+}
+
void __cpuinit kvm_guest_cpu_init(void)
{
if (!kvm_para_available())
@@ -300,11 +320,17 @@ void __cpuinit kvm_guest_cpu_init(void)
smp_processor_id());
}

+ if (kvm_para_has_feature(KVM_FEATURE_PV_EOI)) {
+ __get_cpu_var(kvm_apic_eoi) = 0;
+ wrmsrl(MSR_KVM_PV_EOI_EN, __pa(&__get_cpu_var(kvm_apic_eoi)) |
+ KVM_MSR_ENABLED);
+ }
+
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;
@@ -316,11 +342,18 @@ 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_PV_EOI))
+ wrmsrl(MSR_KVM_PV_EOI_EN, 0);
+ 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;
}

@@ -371,7 +404,9 @@ 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_PV_EOI))
+ wrmsrl(MSR_KVM_PV_EOI_EN, 0);
+ kvm_pv_disable_apf();
apf_task_wake_all();
}

@@ -424,6 +459,16 @@ void __init kvm_guest_init(void)
pv_time_ops.steal_clock = kvm_steal_clock;
}

+ if (kvm_para_has_feature(KVM_FEATURE_PV_EOI)) {
+ struct apic **drv;
+
+ for (drv = __apicdrivers; drv < __apicdrivers_end; drv++) {
+ /* Should happen once for each apic */
+ WARN_ON((*drv)->eoi_write == kvm_guest_apic_eoi_write);
+ (*drv)->eoi_write = kvm_guest_apic_eoi_write;
+ }
+ }
+
#ifdef CONFIG_SMP
smp_ops.smp_prepare_boot_cpu = kvm_smp_prepare_boot_cpu;
register_cpu_notifier(&kvm_cpu_notifier);
--
MST