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

[Julien Grall <julien.grall@xxxxxxx>]

Hi,

On 01/08/17 00:53, Stefano Stabellini wrote:
> 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?
Per Table D4-44 in ARM DDI 0487B.a, outer-shareable takes precedence on 
inner-shareable. So if the guest configures the stage-1 page table with 
outer-shareable, the resultant will be outer-shareable.
As I pointed out in a previous version, this option is a bit confusing 
because the value set in stage-2 does not mean this will be the 
resultant value. I would strongly recommend any user wanting to share 
memory between guests to read D4.5.4 "Combining the stage 1 and stage 2 
attributes" before using this option. Otherwise, he/she will likely be 
confused and end up to mis-configure the attribute.
The current configuration (i.e "ARM_normal") gives the most flexibility 
to the guest in term of memory attribute but prevent non-shareable mapping.
Cheers,

--
Julien Grall

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