Re: [PATCH 1/1] KVM: x86: avoid unnecessary bitmap allocation when memslot is clean

[Marcelo Tosatti <mtosatti@redhat.com>]

On Mon, Apr 26, 2010 at 06:58:54PM +0900, Takuya Yoshikawa wrote:
> Although we always allocate a new dirty bitmap in x86's get_dirty_log(),
> it is only used as a zero-source of copy_to_user() and freed right after
> that when memslot is clean. This patch uses clear_user() instead of doing
> this unnecessary zero-source allocation.
> 
> Performance improvement: as we can expect easily, the time needed to
> allocate a bitmap is completely reduced. In my test, the improved ioctl
> was about 4 to 10 times faster than the original one for clean slots.
> Furthermore, the reduced allocations seem to produce good effects for
> other cases too. Actually, I observed that the time for the ioctl was
> more stable than the original one and the average time for dirty slots
> was also reduced by some extent.
> 
> Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>

Looks good to me.