Re: [Xen-devel] [PATCH v3 0/5] xen/arm: introduce GNTTABOP_cache_flush

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Wed, 8 Oct 2014, David Vrabel wrote:
> On 08/10/14 13:58, Stefano Stabellini wrote:
> > Hi all,
> > this patch series introduces a new hypercall to perform cache
> > maintenance operations on behalf of the guest. It is useful for dom0 to
> > be able to cache flush pages involved in a dma operation with
> > non-coherent devices.
> 
> I was going to suggest requiring the use of the SWIOTLB for all foreign
> pages to non-coherent devices but Ian tells me you've already tried this
> and it performs worse than this proposal.  Is this the case?
> 
> Using the SWIOTLB would require no Xen changes (except for the revert)
> which might be sensible at this point in the Xen 4.5 release cycle.

I benchmarked the swiotlb solution a long time ago and was the worst of
the lot. Back then I didn't try to narrow down its usage only to
non-coherent devices though because it is actually quite an hack and I
hadn't climbed down the ivory tower yet. At this point anything goes.

I have tried that approach now, and it is still the worst of the lot. It
is slower than the hypercall approach by 37% (485 timer ticks on average
vs 352). 

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