RE: [PATCH V3,net-next, 1/2] hv_netvsc: Add XDP support

[Haiyang Zhang <haiyangz@microsoft.com>]

> -----Original Message-----
> From: Jakub Kicinski <kuba@kernel.org>
> Sent: Thursday, January 23, 2020 12:30 PM
> To: Haiyang Zhang <haiyangz@microsoft.com>
> Cc: sashal@kernel.org; linux-hyperv@vger.kernel.org; netdev@vger.kernel.org;
> KY Srinivasan <kys@microsoft.com>; Stephen Hemminger
> <sthemmin@microsoft.com>; olaf@aepfle.de; vkuznets
> <vkuznets@redhat.com>; davem@davemloft.net; linux-kernel@vger.kernel.org
> Subject: Re: [PATCH V3,net-next, 1/2] hv_netvsc: Add XDP support
> 
> On Thu, 23 Jan 2020 17:14:06 +0000, Haiyang Zhang wrote:
> > > > Changes:
> > > > 	v3: Minor code and comment updates.
> > > >         v2: Added XDP_TX support. Addressed review comments.
> > >
> > > How does the locking of the TX path work? You seem to be just
> > > calling the normal xmit method, but you don't hold the xmit queue
> > > lock, so the stack can start xmit concurrently, no?
> >
> > The netvsc and vmbus can handle concurrent transmits, except the msd
> > (Multi-Send Data) field which can only be used by one queue.
> >
> > I already added a new flag to netvsc_send(), so packets from XDP_TX
> > won't use the msd.
> 
> I see, there's a few non-atomic counters there, but maybe that's not a big deal.
Yes, those error counters are used less frequently, and not necessary to be precise.

> What frees the skb if the ring is full, and netvsc_send_pkt() returns -EAGAIN?
> Nothing checks the return value from netvsc_xdp_xmit().
Good catch! I will add skb free when -EAGAIN is returned for XDP_TX.
I will update the patch.

Thanks,
- Haiyang