Linux Virtualization - Mar 2018 - [PATCH net-next 0/2] virtio-net: re enable XDP_REDIRECT for mergeable buffer

On 2018?03?01? 17:10, Jesper Dangaard Brouer wrote:
> On Thu,  1 Mar 2018 11:19:03 +0800
> Jason Wang <jasowang at redhat.com> wrote:
>
>> This series tries to re-enable XDP_REDIRECT for mergeable buffer which
>> was removed since commit 7324f5399b06 ("virtio_net: disable
>> XDP_REDIRECT in receive_mergeable() case"). Main concerns are:
>>
>> - not enough tailroom was reserved which breaks cpumap
> To address this at a more fundamental level, I would suggest that we/you
> instead extend XDP to know it's buffers "frame" size/end. 
(The
> assumption use to be, xdp_buff->data_hard_start + PAGE_SIZE, but
> ixgbe+virtio_net broke that assumption).
>
> It should actually be fairly easy to implement:
>   * Simply extend xdp_buff with a "data_hard_end" pointer.
Right, and then cpumap can warn and drop packets with insufficient 
tailroom. But it should be a patch on top of this I think.

Thanks
>
> Now cpumap is more safe... instead of crashing, it can now know/see when
> it is safe to create an SKB using build_skb (could fallback to
> dev_alloc_skb).
>

On Thu, 1 Mar 2018 17:23:37 +0800
Jason Wang <jasowang at redhat.com> wrote:
> On 2018?03?01? 17:10, Jesper Dangaard Brouer wrote:
> > On Thu,  1 Mar 2018 11:19:03 +0800
> > Jason Wang <jasowang at redhat.com> wrote:
> >  
> >> This series tries to re-enable XDP_REDIRECT for mergeable buffer
which
> >> was removed since commit 7324f5399b06 ("virtio_net: disable
> >> XDP_REDIRECT in receive_mergeable() case"). Main concerns
are:
> >>
> >> - not enough tailroom was reserved which breaks cpumap  
>
> > To address this at a more fundamental level, I would suggest that
we/you
> > instead extend XDP to know it's buffers "frame"
size/end.  (The
> > assumption use to be, xdp_buff->data_hard_start + PAGE_SIZE, but
> > ixgbe+virtio_net broke that assumption).
> >
> > It should actually be fairly easy to implement:
> >   * Simply extend xdp_buff with a "data_hard_end" pointer.  
> 
> Right, and then cpumap can warn and drop packets with insufficient 
> tailroom.
>
> But it should be a patch on top of this I think.
Hmmm, not really.  If we/you instead fix the issue of XDP doesn't know
the end/size of the frame, then we don't need this mixed XDP
generic/native code path mixing.

You could re-enable native redirect, and push the responsibility to
cpumap for detecting this too-small frame "missing tailroom" (and
avoid
crashing...). (If we really want to support this, cpumap could fallback
to dev_alloc_skb, and handle it gracefully).

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer

On 2018?03?01? 18:35, Jesper Dangaard Brouer wrote:
> On Thu, 1 Mar 2018 17:23:37 +0800
> Jason Wang <jasowang at redhat.com> wrote:
>
>> On 2018?03?01? 17:10, Jesper Dangaard Brouer wrote:
>>> On Thu,  1 Mar 2018 11:19:03 +0800
>>> Jason Wang <jasowang at redhat.com> wrote:
>>>   
>>>> This series tries to re-enable XDP_REDIRECT for mergeable
buffer which
>>>> was removed since commit 7324f5399b06 ("virtio_net:
disable
>>>> XDP_REDIRECT in receive_mergeable() case"). Main concerns
are:
>>>>
>>>> - not enough tailroom was reserved which breaks cpumap
>>> To address this at a more fundamental level, I would suggest that
we/you
>>> instead extend XDP to know it's buffers "frame"
size/end.  (The
>>> assumption use to be, xdp_buff->data_hard_start + PAGE_SIZE, but
>>> ixgbe+virtio_net broke that assumption).
>>>
>>> It should actually be fairly easy to implement:
>>>    * Simply extend xdp_buff with a "data_hard_end"
pointer.
>> Right, and then cpumap can warn and drop packets with insufficient
>> tailroom.
>>
>> But it should be a patch on top of this I think.
> Hmmm, not really.  If we/you instead fix the issue of XDP doesn't know
> the end/size of the frame, then we don't need this mixed XDP
> generic/native code path mixing.
I know this but I'm still a little bit confused. According to the commit 
log of 7324f5399b06 ("virtio_net: disable XDP_REDIRECT in 
receive_mergeable() case"), you said:

"""
 ??? The longer explaination is that receive_mergeable() tries to
 ??? work-around and satisfy these XDP requiresments e.g. by having a
 ??? function xdp_linearize_page() that allocates and memcpy RX buffers
 ??? around (in case packet is scattered across multiple rx buffers).? This
 ??? does currently satisfy XDP_PASS, XDP_DROP and XDP_TX (but only because
 ??? we have not implemented bpf_xdp_adjust_tail yet).
"""

So I consider the tailroom is a must for the (future) tail adjustment.
>
> You could re-enable native redirect, and push the responsibility to
> cpumap for detecting this too-small frame "missing tailroom" (and
avoid
> crashing...). (If we really want to support this, cpumap could fallback
> to dev_alloc_skb, and handle it gracefully).
>
Right but it will be slower than build_skb().

Thanks

On Thu, Mar 01, 2018 at 11:35:31AM +0100, Jesper Dangaard Brouer
wrote:
> On Thu, 1 Mar 2018 17:23:37 +0800
> Jason Wang <jasowang at redhat.com> wrote:
> 
> > On 2018?03?01? 17:10, Jesper Dangaard Brouer wrote:
> > > On Thu,  1 Mar 2018 11:19:03 +0800
> > > Jason Wang <jasowang at redhat.com> wrote:
> > >  
> > >> This series tries to re-enable XDP_REDIRECT for mergeable
buffer which
> > >> was removed since commit 7324f5399b06 ("virtio_net:
disable
> > >> XDP_REDIRECT in receive_mergeable() case"). Main
concerns are:
> > >>
> > >> - not enough tailroom was reserved which breaks cpumap  
> >
> > > To address this at a more fundamental level, I would suggest that
we/you
> > > instead extend XDP to know it's buffers "frame"
size/end.  (The
> > > assumption use to be, xdp_buff->data_hard_start + PAGE_SIZE,
but
> > > ixgbe+virtio_net broke that assumption).
> > >
> > > It should actually be fairly easy to implement:
> > >   * Simply extend xdp_buff with a "data_hard_end"
pointer.
> > 
> > Right, and then cpumap can warn and drop packets with insufficient 
> > tailroom.
> >
> > But it should be a patch on top of this I think.
> 
> Hmmm, not really.  If we/you instead fix the issue of XDP doesn't know
> the end/size of the frame, then we don't need this mixed XDP
> generic/native code path mixing.
> 
> You could re-enable native redirect, and push the responsibility to
> cpumap for detecting this too-small frame "missing tailroom" (and
avoid
> crashing...). (If we really want to support this, cpumap could fallback
> to dev_alloc_skb, and handle it gracefully).
Yea, we probably should.

However it's not nice that redirect is now gone in net.
IMHO a smaller version of patch 1/2 (without using generic code)
should go into net. tailroom tracking and fallback to dev_alloc_skb
can go into net-next.
> -- 
> Best regards,
>   Jesper Dangaard Brouer
>   MSc.CS, Principal Kernel Engineer at Red Hat
>   LinkedIn: http://www.linkedin.com/in/brouer