netdev - Re: [PATCH RFC v5 net-next 4/6] virtio-net: add basic interrupt coalescing support

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Date:	Mon, 16 Feb 2015 13:37:53 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Pawel Moll <pawel.moll@....com>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>,
	Jason Wang <jasowang@...hat.com>,
	"netdev\@vger.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	"virtualization\@lists.linux-foundation.org" 
	<virtualization@...ts.linux-foundation.org>,
	"pagupta\@redhat.com" <pagupta@...hat.com>,
	Cornelia Huck <cornelia.huck@...ibm.com>
Subject: Re: [PATCH RFC v5 net-next 4/6] virtio-net: add basic interrupt coalescing support

Pawel Moll <pawel.moll@....com> writes:
> On Fri, 2015-02-13 at 02:52 +0000, Rusty Russell wrote:
>> "Michael S. Tsirkin" <mst@...hat.com> writes:
>> > On Tue, Feb 10, 2015 at 12:02:37PM +1030, Rusty Russell wrote:
>> >> Jason Wang <jasowang@...hat.com> writes:
>> >> > This patch enables the interrupt coalescing setting through ethtool.
>> >> 
>> >> The problem is that there's nothing network specific about interrupt
>> >> coalescing.  I can see other devices wanting exactly the same thing,
>> >> which means we'd deprecate this in the next virtio standard.
>> >> 
>> >> I think the right answer is to extend like we did with
>> >> vring_used_event(), eg:
>> >> 
>> >> 1) Add a new feature VIRTIO_F_RING_COALESCE.
>> >> 2) Add another a 32-bit field after vring_used_event(), eg:
>> >>         #define vring_used_delay(vr) (*(u32 *)((vr)->avail->ring[(vr)->num + 2]))
>> >> 
>> >> This loses the ability to coalesce by number of frames, but we can still
>> >> do number of sg entries, as we do now with used_event, and we could
>> >> change virtqueue_enable_cb_delayed() to take a precise number if we
>> >> wanted.
>> >
>> > But do we expect delay to be update dynamically?
>> > If not, why not stick it in config space?
>> 
>> Hmm, we could update it dynamically (and will, in the case of ethtool).
>> But it won't be common, so we could append a field to
>> virtio_pci_common_cfg for PCI.
>> 
>> I think MMIO and CCW would be easy to extend too, but CC'd to check.
>
> As far as I understand the virtio_pci_common_cfg principle (just had a
> look, for the first time ;-), it's now an equivalent of the MMIO control
> registers block. I see no major problem with adding another one.

OK, thanks.

> Or were you thinking about introducing some standard for the "real"
> config space? (fine with me as well - the transport will have nothing to
> do :-)

No, that'd not be possible at this point.  I think it's a per-transport
decision.

Cheers,
Rusty.
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