netdev - Re: Network performance with small packets

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Date:	Thu, 14 Apr 2011 20:58:41 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	habanero@...ux.vnet.ibm.com, Shirley Ma <mashirle@...ibm.com>,
	Krishna Kumar2 <krkumar2@...ibm.com>,
	David Miller <davem@...emloft.net>, kvm@...r.kernel.org,
	netdev@...r.kernel.org, steved@...ibm.com,
	Tom Lendacky <tahm@...ux.vnet.ibm.com>, borntraeger@...ibm.com
Subject: Re: Network performance with small packets

On Tue, 12 Apr 2011 23:01:12 +0300, "Michael S. Tsirkin" <mst@...hat.com> wrote:
> On Thu, Mar 10, 2011 at 12:19:42PM +1030, Rusty Russell wrote:
> > Here's an old patch where I played with implementing this:
> 
> ...
> 
> > 
> > virtio: put last_used and last_avail index into ring itself.
> > 
> > Generally, the other end of the virtio ring doesn't need to see where
> > you're up to in consuming the ring.  However, to completely understand
> > what's going on from the outside, this information must be exposed.
> > For example, if you want to save and restore a virtio_ring, but you're
> > not the consumer because the kernel is using it directly.
> > 
> > Fortunately, we have room to expand:
> 
> This seems to be true for x86 kvm and lguest but is it true
> for s390?

Yes, as the ring is page aligned so there's always room.

> Will this last bit work on s390?
> If I understand correctly the memory is allocated by host there?

They have to offer the feature, so if the have some way of allocating
non-page-aligned amounts of memory, they'll have to add those extra 2
bytes.

So I think it's OK...
Rusty.
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