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

[Oleksandr Andrushchenko <andr2000@xxxxxxxxx>]

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
> 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)
> , 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.
> Joao
Thank you,
Oleksandr

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