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

From: Paolo Bonzini
Date: Mon Jun 09 2014 - 08:59:46 EST


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@xxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
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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