Re: [Xen-devel] netback grant copying issues

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 07.09.15 at 16:47, <wei.liu2@xxxxxxxxxx> wrote:
> On Fri, Sep 04, 2015 at 03:27:42AM -0600, Jan Beulich wrote:
>> Ian, Wei,
>> 
>> I seem to be seeing two issues in the grant copy handling of netback,
>> solely from code inspection:
>> 
>> 1) Shouldn't MAX_GRANT_COPY_OPS, to take care of the copying
>> the header may require, be
>> ((MAX_SKB_FRAGS + 1) * XEN_NETIF_RX_RING_SIZE)?
>> 
> 
> Not sure what you mean by "header". I take it you mean SKB itself? We do
> need to use grant copy data inside SKB (not in frag list) to frontend.

Yes - with "header" is mean everything up to skb_headlen(skb).

> It sounds plausible that there is some sort of miscounting, just that no
> SKB is seen to be so broken to trigger that.

Why "broken"?

> With that in mind, even MAX_SKB_FRAGS + 1 is not enough. It would be
> MAX_SKB_FRAGS + 64K / PAGE_SIZE, i.e. we count the most extreme
> situation that we have 63K data in SKB, and 1 byte in each frag.

No, every component (head + every frag) can contribute
exactly one copy operation to a RX ring slot (larger heads, just
like larger frags) would get copied into multiple RX ring slots.

Jan


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel