netdev - Re: [PATCH V3,net-next, 1/2] hv

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Message-ID: <20200123093013.53d78485@cakuba>
Date:   Thu, 23 Jan 2020 09:30:13 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Haiyang Zhang <haiyangz@...rosoft.com>
Cc:     "sashal@...nel.org" <sashal@...nel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        KY Srinivasan <kys@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "olaf@...fle.de" <olaf@...fle.de>, vkuznets <vkuznets@...hat.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.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.

What frees the skb if the ring is full, and netvsc_send_pkt() returns
-EAGAIN? Nothing checks the return value from netvsc_xdp_xmit().