Re: [RFC PATCH 2/2] macvtap: TX zero copy between guest and host kernel

Avi Kivity
2010-09-14T09:12:39Z

On 09/14/2010 05:17 AM, David Miller wrote:
> From: Shirley Ma<mashirle@us.ibm.com>
> Date: Mon, 13 Sep 2010 13:48:03 -0700
>
>> + base = (unsigned long)from->iov_base + offset1;
>> + size = ((base& ~PAGE_MASK) + len + ~PAGE_MASK)>> PAGE_SHIFT;
>> + num_pages = get_user_pages_fast(base, size, 0,&page[i]);
>> + if ((num_pages != size) ||
>> + (num_pages> MAX_SKB_FRAGS - skb_shinfo(skb)->nr_frags))
>> + /* put_page is in skb free */
>> + return -EFAULT;
> What keeps the user from writing to these pages in it's address space
> after the write call returns?
>
> A write() return of success means:
>
> "I wrote what you gave to me"
>
> not
>
> "I wrote what you gave to me, oh and BTW don't touch these
> pages for a while."
>
> In fact "a while" isn't even defined in any way, as there is no way
> for the write() invoker to know when the networking card is done with
> those pages.

That's what io_submit() is for. Then io_getevents() tells you what "a
while" actually was.

--
error compiling committee.c: too many arguments to function