linux-kernel - [RFC PATCH 5/5] KVM: x86: speed up emulated moves

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Message-Id: <1395919838-18466-6-git-send-email-pbonzini@redhat.com>
Date:	Thu, 27 Mar 2014 12:30:38 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	linux-kernel@...r.kernel.org
Cc:	kvm@...r.kernel.org
Subject: [RFC PATCH 5/5] KVM: x86: speed up emulated moves

We can just blindly move all 16 bytes of ctxt->src's value to ctxt->dst.
write_register_operand will take care of writing only the lower bytes.

Avoiding a call to memcpy (the compiler optimizes it out) gains about 50
cycles on kvm-unit-tests for register-to-register moves, and makes them
about as fast as arithmetic instructions.

We could perhaps get a larger speedup by moving all instructions _except_
moves out of x86_emulate_insn, removing opcode_len, and replacing the
switch statement with an inlined em_mov.

Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/include/asm/kvm_emulate.h | 2 +-
 arch/x86/kvm/emulate.c             | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/kvm_emulate.h b/arch/x86/include/asm/kvm_emulate.h
index 24ec1216596e..f7b1e45eb753 100644
--- a/arch/x86/include/asm/kvm_emulate.h
+++ b/arch/x86/include/asm/kvm_emulate.h
@@ -232,7 +232,7 @@ struct operand {
 	union {
 		unsigned long val;
 		u64 val64;
-		char valptr[sizeof(unsigned long) + 2];
+		char valptr[sizeof(sse128_t)];
 		sse128_t vec_val;
 		u64 mm_val;
 		void *data;
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 94974055d906..4a3584d419e5 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -2955,7 +2955,7 @@ static int em_rdpmc(struct x86_emulate_ctxt *ctxt)
 
 static int em_mov(struct x86_emulate_ctxt *ctxt)
 {
-	memcpy(ctxt->dst.valptr, ctxt->src.valptr, ctxt->op_bytes);
+	memcpy(ctxt->dst.valptr, ctxt->src.valptr, sizeof(ctxt->src.valptr));
 	return X86EMUL_CONTINUE;
 }
 
-- 
1.8.3.1

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