Re: [RFC PATCH 3/3] KVM: MMU: Optimize guest page table walk

[Joerg Roedel <joro@8bytes.org>]

On Tue, Apr 19, 2011 at 03:38:14AM +0900, Takuya Yoshikawa wrote:
> From: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
> 
> We optimize multi level guest page table walk as follows:
> 
>   1. We cache the memslot which, probably, includes the next guest page
>      tables to avoid searching for it many times.
>   2. We use get_user() instead of copy_from_user().
> 
> Note that this is kind of a restricted way of Xiao's more generic
> work: "KVM: optimize memslots searching and cache GPN to GFN."
> 
> With this patch applied, paging64_walk_addr_generic() has improved
> as the following tracing results show.
> 
> Before:
>   3.169 us   |  paging64_walk_addr_generic();
>   1.880 us   |  paging64_walk_addr_generic();
>   1.243 us   |  paging64_walk_addr_generic();
>   1.517 us   |  paging64_walk_addr_generic();
>   3.009 us   |  paging64_walk_addr_generic();
>   1.814 us   |  paging64_walk_addr_generic();
>   1.340 us   |  paging64_walk_addr_generic();
>   1.659 us   |  paging64_walk_addr_generic();
>   1.748 us   |  paging64_walk_addr_generic();
>   1.488 us   |  paging64_walk_addr_generic();
> 
> After:
>   1.714 us   |  paging64_walk_addr_generic();
>   0.806 us   |  paging64_walk_addr_generic();
>   0.664 us   |  paging64_walk_addr_generic();
>   0.619 us   |  paging64_walk_addr_generic();
>   0.645 us   |  paging64_walk_addr_generic();
>   0.605 us   |  paging64_walk_addr_generic();
>   1.388 us   |  paging64_walk_addr_generic();
>   0.753 us   |  paging64_walk_addr_generic();
>   0.594 us   |  paging64_walk_addr_generic();
>   0.833 us   |  paging64_walk_addr_generic();

Nice optimization! What scenarios have you used to test it?

	Joerg