Re: [RFC PATCH 1/4] KVM: emulate: speed up do_insn_fetch

[Bandan Das]

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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