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

[Joao Martins <joao.m.martins@xxxxxxxxxx>]

On 07/31/2017 11:37 AM, Oleksandr Andrushchenko wrote:
> 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.

Sounds like it is addressed by PV-IOMMU[0] which I think it will be resurrected
in the coming months as per the design session last hackaton.

[0] https://schd.ws/hosted_files/xendeveloperanddesignsummit2017/91/PV-IOMMU.txt

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