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

[Avi Kivity]

 On 09/14/2010 05:17 AM, David Miller wrote:
> From: Shirley Ma<mashirle@xxxxxxxxxx>
> 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

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/