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

[Julien Grall <julien.grall@xxxxxxx>]

Hi,

On 01/08/17 20:07, Stefano Stabellini wrote:
> On Tue, 1 Aug 2017, Oleksandr Andrushchenko wrote:
> > Hi, Stefano!
> >
> > On 07/31/2017 11:28 PM, Stefano Stabellini wrote:
> > > On Mon, 31 Jul 2017, Oleksandr Andrushchenko wrote:
> > > > 3 Sharing with page exchange (XENMEM_exchange)
> > > > ==============================================
> > > >
> > > > This API was pointed to me by Stefano Stabellini as one of the possible
> > > > ways
> > > > to
> > > > achieve zero copying and share physically contiguous buffers. It is used
> > > > by
> > > > x86
> > > > SWIOTLB code (xen_create_contiguous_region, [5]), but as per my
> > > > understanding
> > > > this API cannot be used on ARM as of now [6].  Conclusion: not an option
> > > > for
> > > > ARM
> > > > at the moment
> > > Let me elaborate on this. The purpose of XENMEM_exchange is to exchange
> > > a number of memory pages with an equal number of contiguous memory
> > > pages, possibly even under 4G. The original purpose of the hypercall was
> > > to get DMA-able memory.
> > this is good to know
> > > So far, it has only been used by Dom0 on x86. Dom0 on ARM doesn't need
> > > it because it is mapped 1:1 by default and device assignment is not
> > > allowed without an IOMMU. However it should work on ARM too, as the
> > > implementation is all common code in Xen.
> > well, according to [6]:
> > "Currently XENMEM_exchange is not supported on ARM because the steal_page is
> > left unimplemented.
> >
> > However, even if steal_page is implemented, the hypercall can't work for ARM
> > because:
> >     - Direct mapped domain is not supported
> >     - ARM doesn't have a M2P and therefore usage of mfn_to_gmfn is
> >     invalid"
> > And what I see at [7] is that it is still EOPNOTSUPP
> > So, yes, common code is usable for both ARM and x86, but
> > underlying support for ARM is till not there.
> > Please correct me if I am wrong here
> Ops, I forgot about that! Implementing steal_page on ARM is not be a
> problem, and direct mapped domains are not a concern in this scenario.
>
> The issue is mfn_to_gmfn. However, we do not actually need mfn_to_gmfn
> to implement xen/common/memory.c:memory_exchange as Julien pointed out
> in http://marc.info/?l=xen-devel&m=145037009127660.
>
> Julien, Jan, two years have passed. Do you think we can find a way to
> make that old series work for everybody?
I looked at the series and AFAICT Jan was pushing towards M2P.

I've already made my arguments in another thread (see [1]) why I don't 
think M2P is a solution for ARM.
I really want to kill the use of M2P on ARM because I think this using 
too much memory for the benefits it brings.
I don't have much time to investigate more on XENMEM_exchange, so I 
would be grateful if someone pick up that task.
Cheers,

[1] 
https://lists.xenproject.org/archives/html/xen-devel/2017-05/msg01370.html
--
Julien Grall

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