Re: [PATCH 2/2] KVM: x86 emulator: Avoid clearing the whole decode_cache

[Avi Kivity <avi@redhat.com>]

On 05/25/2011 05:09 AM, Takuya Yoshikawa wrote:
> From: Takuya Yoshikawa<yoshikawa.takuya@oss.ntt.co.jp>
>
> During tracing the emulator, we noticed that init_emulate_ctxt()
> sometimes took a bit longer time than we expected.
>
> This patch is for mitigating the problem by some degree.
>
> By looking into the function, we soon notice that it clears the whole
> decode_cache whose size is about 2.5K bytes now.  Furthermore, most of
> the bytes are taken for the two read_cache arrays, which are used only
> by a few instructions.
>
> Considering the fact that we are not assuming the cache arrays have
> been cleared when we store actual data, we do not need to clear the
> arrays: 2K bytes elimination.  In addition, we can avoid clearing the
> fetch_cache and regs arrays.
>
> This patch changes the initialization not to clear the arrays.
>
> On our 64-bit host, init_emulate_ctxt() becomes 0.3 to 0.5us faster with
> this patch applied.
>

Thanks, applied.

It strikes me that initializing the emulator in x86.c is the wrong 
thing.  We should move the entire thing to x86_decode_insn().

We'll need a few more callbacks for that, though (register, eflags); 
eventually we can read just registers that are used and write only 
registers that were updated.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.