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

[David Woodhouse <dwmw2@infradead.org>]

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On Fri, 2023-10-27 at 07:27 +0200, Juergen Gross wrote:
> On 26.10.23 22:56, Stefano Stabellini wrote:
> > On Thu, 26 Oct 2023, David Woodhouse wrote:
> > > 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.
> > 
> > I think there are two questions here. One question is "Why do we need
> > all the new grant mapping code added to xen-mapcache.c in patch #7?
> > Can't we use qemu_xen_gnttab_map_refs() instead?"
> 
> The main motivation was to _avoid_ having to change all the backends.
> 
> My implementation enables _all_ qemu based virtio backends to use grant
> mappings. And if a new backend is added to qemu, there will be no change
> required to make it work with grants.

I'm not really convinced I buy that. This is a lot of complexity, and
don't backends need to call an appropriate mapping function to map via
an IOMMU if it's present anyway? Make then call a helper where you can
do this in one place directly instead of through a fake MemoryRegion,
and you're done, surely? 

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