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

[Oleksandr Andrushchenko <andr2000@xxxxxxxxx>]

On 07/31/2017 02:58 PM, Joao Martins wrote:
> On 07/31/2017 12:41 PM, Oleksandr Andrushchenko wrote:
> > Hi, Joao!
> >
> > On 07/31/2017 02:03 PM, Joao Martins wrote:
> > > Hey Oleksandr,
> > >
> > > On 07/31/2017 09:34 AM, Oleksandr Andrushchenko wrote:
> > > > Hi, all!
> > > [snip]
> > > > Comparison for display use-case
> > > > ===============================
> > > >
> > > > 1 Number of grant references used
> > > > 1-1 grant references: nr_pages
> > > > 1-2 GNTTABOP_transfer: nr_pages
> > > > 1-3 XENMEM_exchange: not an option
> > > >
> > > > 2 Effect of DomU crash on Dom0 (its mapped pages)
> > > > 2-1 grant references: pages can be unmapped by Dom0, Dom0 is fully
> > > > recovered
> > > > 2-2 GNTTABOP_transfer: pages will be returned to the Hypervisor, lost
> > > > for Dom0
> > > > 2-3 XENMEM_exchange: not an option
> > > >
> > > > 3 Security issues from sharing Dom0 pages to DomU
> > > > 1-1 grant references: none
> > > > 1-2 GNTTABOP_transfer: none
> > > > 1-3 XENMEM_exchange: not an option
> > > >
> > > > At the moment approach 1 with granted references seems to be a winner for
> > > > sharing buffers both ways, e.g. Dom0 -> DomU and DomU -> Dom0.
> > > >
> > > > Conclusion
> > > > ==========
> > > >
> > > > I would like to get some feedback from the community on which approach
> > > > is more
> > > > suitable for sharing large buffers and to have a clear vision on cons
> > > > and pros
> > > > of each one: please feel free to add other metrics I missed and correct
> > > > the ones
> > > > I commented on.  I would appreciate help on comparing approaches 2 and 3
> > > > as I
> > > > have little knowledge of these APIs (2 seems to be addressed by
> > > > Christopher, and
> > > > 3 seems to be relevant to what Konrad/Stefano do WRT SWIOTLB).
> > > Depending on your performance/memory requirements - there could be another
> > > option which is to keep the guest mapped on Domain-0 (what was discussed with
> > > Zerogrant session[0][1] that will be formally proposed in the next month or so).
> > Unfortunately I missed that session during the Summit
> > due to overlapping sessions
> Hmm - Zerocopy Rx (Dom0 -> DomU) would indeed be an interesting topic to bring 
> up.
it is, especially for the systems which require physically contiguous
buffers
> > > But that would only solve the grant maps/unmaps/copies done on Domain-0 (given
> > > the numbers you pasted a bit ago, you might not really need to go to such 
> > > extents)
> > >
> > > [0]
> > > http://schd.ws/hosted_files/xendeveloperanddesignsummit2017/05/zerogrant_spec.pdf
> > > [1]
> > > http://schd.ws/hosted_files/xendeveloperanddesignsummit2017/a8/zerogrant_slides.pdf
> > I will read these, thank you for the links
> > > For the buffers allocated on Dom0 and safely grant buffers from Dom0 to DomU
> > > (which I am not so sure it is possible today :()
> > We have this working in our setup for display (we have implemented
> > z-copy with grant references already)
> Allow me to clarify :) I meant "possible to do it in a safely manner", IOW,
> regarding what I mentioned below in following paragraphs. But your answer below
> clarifies on that aspect.
good :)
> > > , maybe a "contract" from DomU
> > > provide a set of transferable pages that Dom0 holds on for each Dom-0 gref
> > > provided to the guest (and assuming this is only a handful couple of guests as
> > > grant table is not that big).
> > It is an option
> > >    IIUC, From what you pasted above on "Buffer
> > > allocated @Dom0" sounds like Domain-0 could quickly ran out of pages/OOM (and
> > > grants), if you're guest is misbehaving/buggy or malicious; *also* domain-0
> > > grant table is a rather finite/small resource (even though you can override the
> > > number of frames in the arguments).
> > Well, you are right. But, we are focusing on embedded appliances,
> > so those systems we use are not that "dynamic" with that respect.
> > Namely: we have fixed number of domains and their functionality
> > is well known, so we can do rather precise assumption on resource
> > usage.
> Interesting! So here I presume backend trusts the frontend.
yes, this is the case. What is more backend can make decision
on if to allow buffer allocation or reject the request
> Cheers,
> Joao

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