Re: [QEMU][PATCHv2 0/8] Xen: support grant mappings.

[David Woodhouse <dwmw2@infradead.org>]

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On Thu, 2023-10-26 at 13:36 -0700, Stefano Stabellini wrote:
> 
> > This seems like a lot of code to replace that simpler option... is
> > there a massive performance win from doing it this way? Would we want
> > to use this trick for the Xen PV backends (qdisk, qnic) *too*? Might it
> > make sense to introduce the simple version and *then* the optimisation,
> > with some clear benchmarking to show the win?
> 
> This is not done for performance but for safety (as in safety
> certifications, ISO 26262, etc.). This is to enable unprivileged virtio
> backends running in a DomU. By unprivileged I mean a virtio backend that
> is unable to map arbitrary memory (the xenforeignmemory interface is
> prohibited).
> 
> The goal is to run Xen on safety-critical systems such as cars,
> industrial robots and more. In this configuration there is no
> traditional Dom0 in the system at all. If you  would like to know more:
> https://www.youtube.com/watch?v=tisljY6Bqv0&list=PLYyw7IQjL-zHtpYtMpFR3KYdRn0rcp5Xn&index=8

Yeah, I understand why we're using grant mappings instead of just
directly having access via foreignmem mappings. That wasn't what I was
confused about.

What I haven't worked out is why we're implementing this through an
automatically-populated MemoryRegion in QEMU, rather than just using
grant mapping ops like we always have.

It seems like a lot of complexity just to avoid calling
qemu_xen_gnttab_map_refs() from the virtio backend.

And if we're going to use this magic region, are we going to start
using it for the Xen PV backends in QEMU too, when they want to map
grant refs? 


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