Re: [Xen-devel] [RFC v4]Proposal to allow setting up shared memory areas between VMs from xl config file

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 31 Jul 2017, Edgar E. Iglesias wrote:
> > > >   @role                 Can only be 'master' or 'slave', it defaults to 
> > > > 'slave'.
> > > > 
> > > >   @prot                 When @role = master, this means the largest set 
> > > > of
> > > >                         stage-2 permission flags that can be granted to 
> > > > the
> > > >                         slave domains.
> > > > When @role = slave, this means the stage-2 permission
> > > >                         flags of the shared memory area.
> > > >                         Currently only 'rw' is supported. If not set. it
> > > >                         defaults to 'rw'.
> > > > 
> > > >   @cache_policy         The stage-2 cacheability/shareability 
> > > > attributes of the
> > > >                         shared memory area. Currently, only two 
> > > > policies are
> > > >                         supported:
> > > >                           * ARM_normal: Only applicable to ARM guests. 
> > > > This
> > > >                                         would mean Inner and Outer 
> > > > Write-Back
> > > >                                         Cacheable, and Inner Shareable.
> > > 
> > > 
> > > Is there a reason not to set this to Outer Shareable?
> > > Again, mainly useful when these pages get shared with devs as well.
> > > 
> > > The guest can always lower it to Inner Shareable via S1 tables if needed.
> > 
> > I don't think we can support memory sharing with devices in this version
> > of the document (see above about GSoC timelines). Normal memory is inner
> > shareable in Xen today, it makes sense to default to that.
> 
> I thought we mapped RAM as Outer shareable to guests but you seem to be right.
> I think we should be mapping all RAM as Outer Shareable and then let the
> guest decide what is Inner and what is Outer via it's S1 tables.
> Right now it would be impossible to be Coherent with a DMA device outside
> of the Inner domain...
> 
> Perhaps we should fix that and then ARM_normal would by itself become Outer.
> If there's agreement I can test it and send a patch.

Today, only device memory is mapped Outer Shareable, while normal memory
is mapped Inner Shareable. I am OK with changing the default in
mfn_to_p2m_entry to Outer Shareable for normal RAM if the change would
make it possible to do coherent DMA with more devices on the platform.
Julien?

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