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

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Date:	Fri, 13 Feb 2015 13:22:09 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Jason Wang <jasowang@...hat.com>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, pagupta@...hat.com,
	Pawel Moll <pawel.moll@....com>,
	Cornelia Huck <cornelia.huck@...ibm.com>
Subject: Re: [PATCH RFC v5 net-next 4/6] virtio-net: add basic interrupt coalescing support

"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.

>> My feeling is that this should be a v1.0-only feature though
>> (eg. feature bit 33).
>
> Yes, e.g. we can't extend config space for legacy virtio pci.

Thanks,
Rusty.
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