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

[Julien Grall <julien.grall@xxxxxxx>]

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?
Cheers,

--
Julien Grall

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel