Ok! Now that you posted your changes, I am getting to understand this
a little :)
Paolo Bonzini <pbonzini@xxxxxxxxxx> writes:
> 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 both the
> compiler and the branch predictor.
>
> 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 42820f5fdd04..c7b625bf0b5d 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -703,51 +703,51 @@ 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)) {
Is this really going to be unlikely ?
> + rc = do_insn_fetch_bytes(ctxt);
> + if (rc != X86EMUL_CONTINUE)
> + return rc;
> + }
> + *dest++ = *src++;
> + ctxt->_eip++;
> + continue;
> }
> return X86EMUL_CONTINUE;
> }
> @@ -4272,6 +4272,11 @@ 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:
--
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