Re: [Xen-devel] netback grant copying issues

[Wei Liu <wei.liu2@xxxxxxxxxx>]

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.

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

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.

> 2) xenvif_gop_frag_copy() may set up any number of copy_gop-s
> per meta slot, yet xenvif_check_gop() checks only one.
> 

That's indeed something that need to be fixed.

> Apart from that I find it puzzling that the comment ahead of
> xenvif_gop_frag_copy() talks about flipping interfaces, while iirc
> all flipping code has long been deleted from the upstream driver.
> 

That should be removed IMO.

Wei.

> Jan

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