KVM: VMX: FIXED+PHYSICAL mode single target IPI fastpath

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 introduce a mechanism to handle certain performance-critical
WRMSRs in a very early stage of KVM VMExit handler.

This mechanism is specifically used for accelerating writes to x2APIC ICR
that attempt to send a virtual IPI with physical destination-mode, fixed
delivery-mode and single target. Which was found as one of the main causes
of VMExits for Linux workloads.

The reason this mechanism significantly reduce the latency of such virtual
IPIs is by sending the physical IPI to the target vCPU in a very early stage
of KVM VMExit handler, before host interrupts are enabled and before expensive
operations such as reacquiring KVM’s SRCU lock.
Latency is reduced even more when KVM is able to use APICv posted-interrupt
mechanism (which allows to deliver the virtual IPI directly to target vCPU
without the need to kick it to host).

Testing on Xeon Skylake server:

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

Reviewed-by: Liran Alon <liran.alon@oracle.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Liran Alon <liran.alon@oracle.com>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 9 insertions, 2 deletions

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 159a28512e4c..e2b793477243 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -175,6 +175,11 @@ enum {
 	VCPU_SREG_LDTR,
 };
 
+enum exit_fastpath_completion {
+	EXIT_FASTPATH_NONE,
+	EXIT_FASTPATH_SKIP_EMUL_INS,
+};
+
 #include <asm/kvm_emulate.h>
 
 #define KVM_NR_MEM_OBJS 40
@@ -1095,7 +1100,8 @@ 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,
+		enum exit_fastpath_completion exit_fastpath);
 	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);
@@ -1145,7 +1151,8 @@ 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,
+		enum exit_fastpath_completion *exit_fastpath);
 	bool (*mpx_supported)(void);
 	bool (*xsaves_supported)(void);
 	bool (*umip_emulated)(void);