Re: [PATCH 0/1] KVM: make get dirty log ioctl return the first dirty page's position

[Avi Kivity <avi@redhat.com>]

On 02/24/2010 11:20 AM, Takuya Yoshikawa wrote:
>
> Another issue I am thinking is the x86's bitmap allocation. Doing 
> vmalloc()
> every time is not nice, though I know it's needed.
>

Right, that's why I want to move the allocation to userspace.

>>
>> btw, one idea I had was to allocate the bitmap in userspace and let 
>> the kernel set bits directly.  This reduces the amount of unswappable 
>> memory the kernel allocates and reduces copying.
>>
>> A problem with this is that userspace cannot just clear the bits, 
>> since the kernel has to write-protect the pages again.  I don't know 
>> how we can do this without copying the bitmap.
>>
>
> Yes, seems difficult.

Here's an idea: double buffering.  Instead of copying the dirty log to 
userspace, we switch the dirty log bitmap to another location 
atomically.  Then userspace can read the old bitmap.

Note the speed savings won't be huge, since the copying brings the 
bitmap into cache, so the second pass is fairly fast.  But if we combine 
it with dropping the qemu byte-based dirty log, then we eliminate the 
second pass as well.

-- 
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