Re: [PATCH RFC 5/5] KVM: This is the main part of the "moving dirty bitmaps to user space"

Fernando Luis Vazquez Cao
2010-04-12T21:18:29Z

Avi Kivity wrote:
> On 04/12/2010 05:29 AM, Takuya Yoshikawa wrote:
>>>> TODO:
>>>> 1. We want to use copy_in_user() for 32bit case too.
>>>
>>> Definitely. Why doesn't it work now?
>>
>> Sadly we don't have that for 32bit. We have to implement by ourselves.
>>
>> I tested two temporary implementations for 32bit:
>> 1. This version using copy_from_user() and copy_to_user() with
>> not nice vmalloc().
>> 2. Loop with __get_user() and __put_user().
>>
>> The result was 1 is much faster than 2.
>
> What about copy_from_user()/copy_to_user() through a 512 byte buffer
> on the kernel stack?

Reserving 512 bytes on the stack looks like too much, I'd rather kmalloc
a 512 byte buffer at VM
creation time and pass it down to the dirty page tracking code. Would
you be OK with such an
approach?