Re: [Xen-devel] [RFC][PATCH 0/5] Add V4V to Xen

[Tim Deegan <tim@xxxxxxx>]

At 11:38 +0100 on 28 Jun (1340883538), Jean Guyader wrote:
> On 26/06 03:38, Ian Campbell wrote:
> > Lastly -- have you given any thoughts to making the copy operation
> > asynchronous? This might allow us to take advantage of copy offload
> > hardware in the future?
> 
> A CPU copy already has to happen once in the guest to put it in the
> ring

I don't understand -- isn't the vector-send operation designed to have
Xen do a single copy from scattered sources into the destination ring?
So the sender could be zero-copy?

> so the second copy that is done in Xen will be really cheap because
> it's very linkely going to be in the cache.  I don't doing async copy
> in Xen will have a huge impact on the performance.

Using a DMA engine to do the copy could free up CPU cycles that Xen
could use for other vcpus, so even if there isn't a throughput increase
on the v4v channel, there could be a benefit to the system as a whole.
We could do that with synchronous copies, pausing the vcpu while the
copy happens, but I think to get full throughput we'd want to pipeline a
few writes ahead.

Anyway, that's all very forward-looking: AFAICS there's no need to
address it right now except to be careful not to bake things into the
interface that would stop us doing this later on.

Cheers,

Tim.

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