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Subject: Re: [virtio-dev] Re: [virtio] [PATCH] ccw: split descriptor/available/used rings


On Tue, 15 Oct 2013 12:53:00 +1030
Rusty Russell <rusty@au1.ibm.com> wrote:

> Back to the actual subject of splitting the rings: we've already removed
> the assumption that they will be contiguous from the core of the spec,
> but that does not mean transports need to do the same.
> 
> The effect is to increase the ring size when large contiguous memory
> ranges can't be obtained (eg. hotplug on long-running kernels).
> 
> (PAGESIZE=4096, ALIGN=4096)
> 
> Contiguous Pages        Max Qsz (unsplit)       Max Qsz (split)
> 1                       None                    256
> 2                       128                     512
> 3                       256                     512
> 4                       512                     1024
> 5                       512                     1024
> 6                       512                     1024
> 7                       1024                    1024
> 8                       1024                    2048
> 
> If this doesn't matter to you, it's almost certainly not worth the
> hassle of changing.

Well, it sounds like a good thing to have, especially since we're
likely to have both long running instances and a lot of hotplugging.

I'd like to extend the communication structure though, like I did in my
alternate proposal. I'll post a v2 without the special command reject
handling.

> 
> Cheers,
> Rusty.
> PS.  I note that you expose KVM_VIRTIO_CCW_RING_ALIGN through uapi: that
> seems unnecessary?

virtio-pci exposes its alignment as well.



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