RFC: cache_regs in kvm_emulate_pio

[Marcelo Tosatti <mtosatti@redhat.com>]

Hi,

>From my understanding the ->cache_regs call on kvm_emulate_pio() is
necessary only on AMD, where vcpu->arch.regs[RAX] is not copied during
exit in svm_vcpu_load().

On both architectures, the remaining general purpose registers are saved
on exit.

The following patch saves 100 cycles out of both light and heavy exits
on Intel (if correct, kvm_emulate_hypercall and complete_pio could also
benefit, thus saving 200 cycles for in-kernel devices).

BTW, the decache_regs(vcpu) call at the end of complete_pio() could also
be a noop on Intel from what I can tell ?


diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 238e8f3..6f247cc 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -709,6 +709,13 @@ static void svm_vcpu_put(struct kvm_vcpu *vcpu)
 	rdtscll(vcpu->arch.host_tsc);
 }
 
+static void svm_cache_rax(struct kvm_vcpu *vcpu)
+{
+	struct vcpu_svm *svm = to_svm(vcpu);
+
+	vcpu->arch.regs[VCPU_REGS_RAX] = svm->vmcb->save.rax;
+}
+
 static void svm_cache_regs(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_svm *svm = to_svm(vcpu);
@@ -1949,6 +1956,7 @@ static struct kvm_x86_ops svm_x86_ops = {
 	.set_gdt = svm_set_gdt,
 	.get_dr = svm_get_dr,
 	.set_dr = svm_set_dr,
+	.cache_rax = svm_cache_rax,
 	.cache_regs = svm_cache_regs,
 	.decache_regs = svm_decache_regs,
 	.get_rflags = svm_get_rflags,
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 6e4278d..0d9a148 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -932,6 +932,13 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, u32 msr_index, u64 data)
 }
 
 /*
+ * vcpu->arch.regs[RAX] already loaded by vmx_vcpu_run().
+ */
+static void vcpu_load_rax(struct kvm_vcpu *vcpu)
+{
+}
+
+/*
  * Sync the rsp and rip registers into the vcpu structure.  This allows
  * registers to be accessed by indexing vcpu->arch.regs.
  */
@@ -3213,6 +3220,7 @@ static struct kvm_x86_ops vmx_x86_ops = {
 	.set_idt = vmx_set_idt,
 	.get_gdt = vmx_get_gdt,
 	.set_gdt = vmx_set_gdt,
+	.cache_rax = vcpu_load_rax,
 	.cache_regs = vcpu_load_rsp_rip,
 	.decache_regs = vcpu_put_rsp_rip,
 	.get_rflags = vmx_get_rflags,
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 26b051b..6111946 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2302,7 +2302,7 @@ int kvm_emulate_pio(struct kvm_vcpu *vcpu, struct kvm_run *run, int in,
 		KVMTRACE_2D(IO_WRITE, vcpu, vcpu->run->io.port, (u32)size,
 			    handler);
 
-	kvm_x86_ops->cache_regs(vcpu);
+	kvm_x86_ops->cache_rax(vcpu);
 	memcpy(vcpu->arch.pio_data, &vcpu->arch.regs[VCPU_REGS_RAX], 4);
 
 	kvm_x86_ops->skip_emulated_instruction(vcpu);
diff --git a/include/asm-x86/kvm_host.h b/include/asm-x86/kvm_host.h
index 851184d..95a0736 100644
--- a/include/asm-x86/kvm_host.h
+++ b/include/asm-x86/kvm_host.h
@@ -410,6 +410,7 @@ struct kvm_x86_ops {
 	unsigned long (*get_dr)(struct kvm_vcpu *vcpu, int dr);
 	void (*set_dr)(struct kvm_vcpu *vcpu, int dr, unsigned long value,
 		       int *exception);
+	void (*cache_rax)(struct kvm_vcpu *vcpu);
 	void (*cache_regs)(struct kvm_vcpu *vcpu);
 	void (*decache_regs)(struct kvm_vcpu *vcpu);
 	unsigned long (*get_rflags)(struct kvm_vcpu *vcpu);