Re: [Xen-devel] [PATCH v6 09/10] tools/arm: tee: add "tee" option for xl.cfg

[Julien Grall <julien.grall@xxxxxxx>]

On 6/18/19 3:30 PM, Volodymyr Babchuk wrote:
> Julien Grall writes:
>
> > On 18/06/2019 12:19, Volodymyr Babchuk wrote:
> > > Hi Julien,
> > Hi,
> >
> > > Julien Grall writes:
> > > > > +
> > > > > +=item B<optee>
> > > > > +
> > > > > +Allow a guest to use OP-TEE. Note that a virtualization-aware OP-TEE
> > > > > +is required for this. If this option is selected, guest will be able
> > > > OOI, what happen if OP-TEE does not support virtualization. Will Xen
> > > > forbid to use it?
> > > Yes, Xen will get an error from OP-TEE during domain construction. This
> > > will lead to domain creation failure.
> > This is a bit odd. It means we have no way to know in advance whether
> > OP-TEE will be able to create a client.
> Yes. There can be at least two reasons for this:
> 1. OP-TEE is built without virtualization support at all
> 2. OP-TEE have no resources for a new guest
>
> > In other word, when the
> > mediator is built in Xen, all existing setup with OP-TEE (and
> > no-virtualization) will fail.
> Right. If user provides DTB with 'optee' node, but OP-TEE is build without
> virtualization support, Dom0 will not be created. This can be fixed by
> adding new capability flag into OP-TEE, that tells Xen about
> virtualization support. For some reason I missed this when I implemented
> VM support in OP-TEE :(
>
> > My expectation is Xen should be able to know whether the mediator can be used.
> I need to implement additional capability flag in the OP-TEE. This is
> not so hard, but it will be available only in the next release. For now,
> we can document this limitation somewhere.
Is OP-TEE already released with virtualization? If not, when will it be?

> > > > > +to access to the real OP-TEE OS running on the host. Guest creation
> > > > s/real// it is redundant with the rest of the sentence. However, it
> > > > does not really answer to the question regarding isolation.
> > > Your assumption is correct - OP-TEE provides isolation on its side.
> > >
> > > > > +will fail if OP-TEE have no resources for a new guest. Number of supported
> > > > > +guests depends on OP-TEE configuration.
> > > > How about the following description (correct me if my understanding is
> > > > wrong):
> > > >
> > > > "Allow a guest to access the host OP-TEE OS. Xen will mediate the
> > > > access to OP-TEE and the resource isolation will be provided directly
> > > > by OP-TEE. OP-TEE itself may limit the number of guests that can
> > > > concurrently use it. This requires a virtualization-aware OP-TEE for
> > > > this to work.
> > > >
> > > > This feature is a B<technology preview>."
> > > That's much better than my version. Thank you.
> > >
> > > > How can a user know whether OP-TEE supports virtualization? Is it
> > > > configurable at build?
> > > Yes, there is a special configuration option CFG_VIRTUALIZATION. This is
> > > covered in OP-TEE documentation at [1]
> > >
> > > [1] https://optee.readthedocs.io/architecture/virtualization.html
> > Do we expect the link to be stable? If so, then I think a link in the
> > documentation would be useful.
> This is the official OP-TEE documentation. So, yes, it should be stable.
> I can put this link into the code somewhere.
I would add the link in the xl documentation and also in the commit 
message of patch #2. I can do the later on commit.
Cheers,

--
Julien Grall

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