Re: PV drivers and zero copying

[Oleksandr Andrushchenko <andr2000@gmail.com>]

On 07/31/2017 12:47 PM, Julien Grall wrote:
>
>
> On 31/07/17 10:46, Oleksandr Andrushchenko wrote:
>> Hi, Julien!
>>
>>
>> On 07/31/2017 12:24 PM, Julien Grall wrote:
>>> (+ Joao)
>>>
>>> On 31/07/17 09:34, Oleksandr Andrushchenko wrote:
>>>> Hi, all!
>>>
>>> Hi Oleksandr,
>>>
>>>> The aim of this mail is to highlight and discuss possible 
>>>> approaches to
>>>> implementing zero copying for PV drivers. Rationale behind this is 
>>>> that
>>>> there
>>>> are use-cases when drivers operate with big shared buffers, e.g.
>>>> display, when
>>>> memory copying from front’s buffer into back’s one may 
>>>> significantly hit
>>>> performance of the system (for example, for para-virtual display 
>>>> running
>>>> at full
>>>> HD resolution at 60Hz it is approximately 475MB/sec).
>>>>
>>>> Assumptions (which actually fit ARM platforms, but can be extended to
>>>> other
>>>> platforms as well): Dom0 is a 1:1 mapped privileged domain, runs 
>>>> backend
>>>> driver/software DomU is an unprivileged domain without 1:1 memory
>>>> mapping, runs
>>>> frontend driver
>>>
>>> I would rather avoid to stick with this assumption on ARM. This was
>>> only meant to be a workaround for platform without IOMMU (see [1]) and
>>> we will get into trouble when using IOMMU.
>> You are correct, thank you
>>>
>>> For instance, there are no requirement to have the IOMMU supporting as
>>> many as address bits than the processor. So 1:1 mapping here will not
>>> be an option.
>>>
>>>>
>>>> Buffer origin: while implementing zero copying the buffer 
>>>> allocation can
>>>> happen
>>>> either on DomU’s end or Dom0’s one depending on the use-case and HW
>>>> capabilities/availability: When DomU allocates: It cannot guarantee
>>>> physical
>>>> memory continuity of the buffers allocated Dom0’s HW *can* handle
>>>> non-contiguous
>>>> memory buffers allocated by DomU for memory operations (DMA, for
>>>> example), e.g.
>>>> either with IOMMU help or by any other means (HW block’s own MMU).  
>>>> When
>>>> Dom0
>>>> allocates as it is mapped 1:1 it can allocate physically contiguous
>>>> memory
>>>> Dom0’s HW *cannot* handle non-contiguous memory buffers allocated by
>>>> DomU for
>>>> memory operations by any means.
>>>
>>> I am not sure to follow this. How zero copy is related to 1:1 mapping?
>>> Is it because you have hardware that does not support scatter/gather
>>> IO or IOMMU?
>> yes, you got it right
>
> 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
> Cheers,
>


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