[PATCH 19/25] KVM: emulate: speed up do_insn_fetch

From: Paolo Bonzini
Date: Mon Jun 09 2014 - 09:01:56 EST


Hoist the common case up from do_insn_fetch_byte to do_insn_fetch,
and prime the fetch_cache in x86_decode_insn. This helps a bit the
compiler and the branch predictor, but above all it lays the
ground for further changes in the next few patches.

Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
arch/x86/kvm/emulate.c | 67 +++++++++++++++++++++++++++-----------------------
1 file changed, 36 insertions(+), 31 deletions(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 32f38a3467a0..aee124a4cc6c 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -704,51 +704,51 @@ static int segmented_read_std(struct x86_emulate_ctxt *ctxt,
}

/*
- * Fetch the next byte of the instruction being emulated which is pointed to
- * by ctxt->_eip, then increment ctxt->_eip.
- *
- * Also prefetch the remaining bytes of the instruction without crossing page
+ * Prefetch the remaining bytes of the instruction without crossing page
* boundary if they are not in fetch_cache yet.
*/
-static int do_insn_fetch_byte(struct x86_emulate_ctxt *ctxt, u8 *dest)
+static int do_insn_fetch_bytes(struct x86_emulate_ctxt *ctxt)
{
struct fetch_cache *fc = &ctxt->fetch;
int rc;
int size, cur_size;
-
- if (ctxt->_eip == fc->end) {
- unsigned long linear;
- struct segmented_address addr = { .seg = VCPU_SREG_CS,
- .ea = ctxt->_eip };
- cur_size = fc->end - fc->start;
- size = min(15UL - cur_size,
- PAGE_SIZE - offset_in_page(ctxt->_eip));
- rc = __linearize(ctxt, addr, size, false, true, &linear);
- if (unlikely(rc != X86EMUL_CONTINUE))
- return rc;
- rc = ctxt->ops->fetch(ctxt, linear, fc->data + cur_size,
- size, &ctxt->exception);
- if (unlikely(rc != X86EMUL_CONTINUE))
- return rc;
- fc->end += size;
- }
- *dest = fc->data[ctxt->_eip - fc->start];
- ctxt->_eip++;
+ unsigned long linear;
+
+ struct segmented_address addr = { .seg = VCPU_SREG_CS,
+ .ea = fc->end };
+ cur_size = fc->end - fc->start;
+ size = min(15UL - cur_size,
+ PAGE_SIZE - offset_in_page(fc->end));
+ if (unlikely(size == 0))
+ return X86EMUL_UNHANDLEABLE;
+ rc = __linearize(ctxt, addr, size, false, true, &linear);
+ if (unlikely(rc != X86EMUL_CONTINUE))
+ return rc;
+ rc = ctxt->ops->fetch(ctxt, linear, fc->data + cur_size,
+ size, &ctxt->exception);
+ if (unlikely(rc != X86EMUL_CONTINUE))
+ return rc;
+ fc->end += size;
return X86EMUL_CONTINUE;
}

static int do_insn_fetch(struct x86_emulate_ctxt *ctxt,
- void *dest, unsigned size)
+ void *__dest, unsigned size)
{
int rc;
+ struct fetch_cache *fc = &ctxt->fetch;
+ u8 *dest = __dest;
+ u8 *src = &fc->data[ctxt->_eip - fc->start];

- /* x86 instructions are limited to 15 bytes. */
- if (unlikely(ctxt->_eip + size - ctxt->eip > 15))
- return X86EMUL_UNHANDLEABLE;
while (size--) {
- rc = do_insn_fetch_byte(ctxt, dest++);
- if (rc != X86EMUL_CONTINUE)
- return rc;
+ if (unlikely(ctxt->_eip == fc->end)) {
+ rc = do_insn_fetch_bytes(ctxt);
+ if (rc != X86EMUL_CONTINUE)
+ return rc;
+ }
+ *dest++ = *src++;
+ ctxt->_eip++;
+ continue;
}
return X86EMUL_CONTINUE;
}
@@ -4301,6 +4301,11 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len)
ctxt->opcode_len = 1;
if (insn_len > 0)
memcpy(ctxt->fetch.data, insn, insn_len);
+ else {
+ rc = do_insn_fetch_bytes(ctxt);
+ if (rc != X86EMUL_CONTINUE)
+ return rc;
+ }

switch (mode) {
case X86EMUL_MODE_REAL:
--
1.8.3.1


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