Re: [Xen-devel] PV drivers and zero copying

[Oleksandr Andrushchenko <andr2000@xxxxxxxxx>]

On 07/31/2017 01:04 PM, Julien Grall wrote:
> On 31/07/17 10:52, Oleksandr Andrushchenko wrote:
> > On 07/31/2017 12:47 PM, Julien Grall wrote:
> > > On 31/07/17 10:46, Oleksandr Andrushchenko wrote:
> > > Do you have any example of hardware? What are the performance you
> > > require with them?
> > Currently our target is Renesas R-Car Gen3
> > At the moment I don't have clean requirements, but
> > ideally, PV driver introduces 0% performance drop
> > Some time soon I will have numbers on running display/GPU
> > with and without zero-copy - will keep updated
> PV driver with 0% performance drop sounds a stretch target.
It is, but we should always get as close as possible
> But this is does not answer to my question. Do you have any hardware 
> that does not support scatter/gather
AFAIK display driver which doesn't support scatter-gather on our platform
(BSP based on 4.9 kernel, rcar_du uses DRM CMA - DRM contiguous memory 
allocator)
Anyways, for pages above 4GB even scatter-gather will not help
devices with 32-bit DMA
> or not protected by an IOMMU that will be interfaced with PV drivers?
As per my understanding, IOMMU is solely owned by the hypervisor now
and there is no API to tell Xen from Dom0 to setup IOMMU for such
a buffer (pages), so display HW can do DMA with that buffer.
Thus, Dom0 has no means to do that work and make PV driver produce
buffers which can be used by the real HW driver without bounce buffering.
> Cheers,

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