[PATCH v2 1/2] KVM: VMX: FIXED+PHYSICAL mode single target IPI fastpath

Wanpeng Li
2019-11-19T06:36:43Z

From: Wanpeng Li <wanpengli@tencent.com>

ICR and TSCDEADLINE MSRs write cause the main MSRs write vmexits in
our product observation, multicast IPIs are not as common as unicast
IPI like RESCHEDULE_VECTOR and CALL_FUNCTION_SINGLE_VECTOR etc.

This patch tries to optimize x2apic physical destination mode, fixed
delivery mode single target IPI by delivering IPI to receiver as soon
as possible after sender writes ICR vmexit to avoid various checks
when possible, especially when running guest w/ --overcommit cpu-pm=on
or guest can keep running, IPI can be injected to target vCPU by
posted-interrupt immediately.

Testing on Xeon Skylake server:

The virtual IPI latency from sender send to receiver receive reduces
more than 200+ cpu cycles.

Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
---
v1 -> v2:
* add tracepoint
* Instead of a separate vcpu->fast_vmexit, set exit_reason
to vmx->exit_reason to -1 if the fast path succeeds.
* move the "kvm_skip_emulated_instruction(vcpu)" to vmx_handle_exit
* moving the handling into vmx_handle_exit_irqoff()

arch/x86/include/asm/kvm_host.h | 4 ++--
arch/x86/include/uapi/asm/vmx.h | 1 +
arch/x86/kvm/svm.c | 4 ++--
arch/x86/kvm/vmx/vmx.c | 40 +++++++++++++++++++++++++++++++++++++---
arch/x86/kvm/x86.c | 5 +++--
5 files changed, 45 insertions(+), 9 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 898ab9e..0daafa9 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1084,7 +1084,7 @@ struct kvm_x86_ops {
void (*tlb_flush_gva)(struct kvm_vcpu *vcpu, gva_t addr);

void (*run)(struct kvm_vcpu *vcpu);
- int (*handle_exit)(struct kvm_vcpu *vcpu);
+ int (*handle_exit)(struct kvm_vcpu *vcpu, u32 *vcpu_exit_reason);
int (*skip_emulated_instruction)(struct kvm_vcpu *vcpu);
void (*set_interrupt_shadow)(struct kvm_vcpu *vcpu, int mask);
u32 (*get_interrupt_shadow)(struct kvm_vcpu *vcpu);
@@ -1134,7 +1134,7 @@ struct kvm_x86_ops {
int (*check_intercept)(struct kvm_vcpu *vcpu,
struct x86_instruction_info *info,
enum x86_intercept_stage stage);
- void (*handle_exit_irqoff)(struct kvm_vcpu *vcpu);
+ void (*handle_exit_irqoff)(struct kvm_vcpu *vcpu, u32 *vcpu_exit_reason);
bool (*mpx_supported)(void);
bool (*xsaves_supported)(void);
bool (*umip_emulated)(void);
diff --git a/arch/x86/include/uapi/asm/vmx.h b/arch/x86/include/uapi/asm/vmx.h
index 3eb8411..b33c6e1 100644
--- a/arch/x86/include/uapi/asm/vmx.h
+++ b/arch/x86/include/uapi/asm/vmx.h
@@ -88,6 +88,7 @@
#define EXIT_REASON_XRSTORS 64
#define EXIT_REASON_UMWAIT 67
#define EXIT_REASON_TPAUSE 68
+#define EXIT_REASON_NEED_SKIP_EMULATED_INSN -1

#define VMX_EXIT_REASONS \
{ EXIT_REASON_EXCEPTION_NMI, "EXCEPTION_NMI" }, \
diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index d02a73a..c8e063a 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -4929,7 +4929,7 @@ static void svm_get_exit_info(struct kvm_vcpu *vcpu, u64 *info1, u64 *info2)
*info2 = control->exit_info_2;
}

-static int handle_exit(struct kvm_vcpu *vcpu)
+static int handle_exit(struct kvm_vcpu *vcpu, u32 *vcpu_exit_reason)
{
struct vcpu_svm *svm = to_svm(vcpu);
struct kvm_run *kvm_run = vcpu->run;
@@ -6187,7 +6187,7 @@ static int svm_check_intercept(struct kvm_vcpu *vcpu,
return ret;
}

-static void svm_handle_exit_irqoff(struct kvm_vcpu *vcpu)
+static void svm_handle_exit_irqoff(struct kvm_vcpu *vcpu, u32 *vcpu_exit_reason)
{

}
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 621142e5..b98198d 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -5792,7 +5792,7 @@ void dump_vmcs(void)
* The guest has exited. See if we can fix it or if we need userspace
* assistance.
*/
-static int vmx_handle_exit(struct kvm_vcpu *vcpu)
+static int vmx_handle_exit(struct kvm_vcpu *vcpu, u32 *vcpu_exit_reason)
{
struct vcpu_vmx *vmx = to_vmx(vcpu);
u32 exit_reason = vmx->exit_reason;
@@ -5878,7 +5878,10 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu)
}
}

- if (exit_reason < kvm_vmx_max_exit_handlers
+ if (*vcpu_exit_reason == EXIT_REASON_NEED_SKIP_EMULATED_INSN) {
+ kvm_skip_emulated_instruction(vcpu);
+ return 1;
+ } else if (exit_reason < kvm_vmx_max_exit_handlers
&& kvm_vmx_exit_handlers[exit_reason]) {
#ifdef CONFIG_RETPOLINE
if (exit_reason == EXIT_REASON_MSR_WRITE)
@@ -6223,7 +6226,36 @@ static void handle_external_interrupt_irqoff(struct kvm_vcpu *vcpu)
}
STACK_FRAME_NON_STANDARD(handle_external_interrupt_irqoff);

-static void vmx_handle_exit_irqoff(struct kvm_vcpu *vcpu)
+static u32 handle_ipi_fastpath(struct kvm_vcpu *vcpu)
+{
+ u32 index;
+ u64 data;
+ int ret = 0;
+
+ if (lapic_in_kernel(vcpu) && apic_x2apic_mode(vcpu->arch.apic)) {
+ /*
+ * fastpath to IPI target, FIXED+PHYSICAL which is popular
+ */
+ index = kvm_rcx_read(vcpu);
+ data = kvm_read_edx_eax(vcpu);
+
+ if (((index - APIC_BASE_MSR) << 4 == APIC_ICR) &&
+ ((data & KVM_APIC_DEST_MASK) == APIC_DEST_PHYSICAL) &&
+ ((data & APIC_MODE_MASK) == APIC_DM_FIXED)) {
+
+ trace_kvm_msr_write(index, data);
+ kvm_lapic_set_reg(vcpu->arch.apic, APIC_ICR2, (u32)(data >> 32));
+ ret = kvm_lapic_reg_write(vcpu->arch.apic, APIC_ICR, (u32)data);
+
+ if (ret == 0)
+ return EXIT_REASON_NEED_SKIP_EMULATED_INSN;
+ }
+ }
+
+ return ret;
+}
+
+static void vmx_handle_exit_irqoff(struct kvm_vcpu *vcpu, u32 *exit_reason)
{
struct vcpu_vmx *vmx = to_vmx(vcpu);

@@ -6231,6 +6263,8 @@ static void vmx_handle_exit_irqoff(struct kvm_vcpu *vcpu)
handle_external_interrupt_irqoff(vcpu);
else if (vmx->exit_reason == EXIT_REASON_EXCEPTION_NMI)
handle_exception_nmi_irqoff(vmx);
+ else if (vmx->exit_reason == EXIT_REASON_MSR_WRITE)
+ *exit_reason = handle_ipi_fastpath(vcpu);
}

static bool vmx_has_emulated_msr(int index)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 991dd01..a53bce3 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7981,6 +7981,7 @@ EXPORT_SYMBOL_GPL(__kvm_request_immediate_exit);
static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
{
int r;
+ u32 exit_reason = 0;
bool req_int_win =
dm_request_for_irq_injection(vcpu) &&
kvm_cpu_accept_dm_intr(vcpu);
@@ -8226,7 +8227,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
vcpu->mode = OUTSIDE_GUEST_MODE;
smp_wmb();

- kvm_x86_ops->handle_exit_irqoff(vcpu);
+ kvm_x86_ops->handle_exit_irqoff(vcpu, &exit_reason);

/*
* Consume any pending interrupts, including the possible source of
@@ -8270,7 +8271,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
kvm_lapic_sync_from_vapic(vcpu);

vcpu->arch.gpa_available = false;
- r = kvm_x86_ops->handle_exit(vcpu);
+ r = kvm_x86_ops->handle_exit(vcpu, &exit_reason);
return r;

cancel_injection:
--
2.7.4