linux-kernel - [PATCH 12/25] KVM: emulate: extend memory access optimization to stores

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Date:	Mon,  9 Jun 2014 14:59:00 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	linux-kernel@...r.kernel.org
Cc:	bdas@...hat.com, gleb@...nel.org
Subject: [PATCH 12/25] KVM: emulate: extend memory access optimization to stores

Even on a store the optimization saves about 50 clock cycles,
mostly because the jump in write_memory_operand becomes much more
predictable.

Reviewed-by: Paolo Bonzini <pbonzini@...hat.com>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/kvm/emulate.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 594cb560947c..eaf0853ffaf9 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -1589,7 +1589,7 @@ static int load_segment_descriptor(struct x86_emulate_ctxt *ctxt,
 
 static int prepare_memory_operand(struct x86_emulate_ctxt *ctxt,
 				  struct operand *op,
-				  bool write)
+				  bool read, bool write)
 {
 	int rc;
 	unsigned long gva;
@@ -1605,6 +1605,10 @@ static int prepare_memory_operand(struct x86_emulate_ctxt *ctxt,
 	if (rc != X86EMUL_CONTINUE)
 		return rc;
 
+	/* optimisation - avoid slow emulated read if Mov */
+	if (!read)
+		return X86EMUL_CONTINUE;
+
 	if (likely(!kvm_is_error_hva(op->hva))) {
 		rc = read_from_user(ctxt, op->hva, &op->val, size);
 		if (!write)
@@ -4699,14 +4703,14 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
 	}
 
 	if ((ctxt->src.type == OP_MEM) && !(ctxt->d & NoAccess)) {
-		rc = prepare_memory_operand(ctxt, &ctxt->src, false);
+		rc = prepare_memory_operand(ctxt, &ctxt->src, true, false);
 		if (rc != X86EMUL_CONTINUE)
 			goto done;
 		ctxt->src.orig_val64 = ctxt->src.val64;
 	}
 
 	if (ctxt->src2.type == OP_MEM) {
-		rc = prepare_memory_operand(ctxt, &ctxt->src2, false);
+		rc = prepare_memory_operand(ctxt, &ctxt->src2, true, false);
 		if (rc != X86EMUL_CONTINUE)
 			goto done;
 	}
@@ -4715,9 +4719,9 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
 		goto special_insn;
 
 
-	if ((ctxt->dst.type == OP_MEM) && !(ctxt->d & Mov)) {
-		/* optimisation - avoid slow emulated read if Mov */
+	if (ctxt->dst.type == OP_MEM) {
 		rc = prepare_memory_operand(ctxt, &ctxt->dst,
+					    !(ctxt->d & Mov),
 					    !(ctxt->d & NoWrite));
 		if (rc != X86EMUL_CONTINUE)
 			goto done;
-- 
1.8.3.1


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