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

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
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
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
either on DomU’s end or Dom0’s one depending on the use-case and HW
capabilities/availability: When DomU allocates: It cannot guarantee
memory continuity of the buffers allocated Dom0’s HW *can* handle
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
allocates as it is mapped 1:1 it can allocate physically contiguous
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
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

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