Re: [Xen-devel] [PATCH v4 06/10] xen/arm: optee: add support for RPC SHM buffers

[Julien Grall <julien.grall@xxxxxxx>]

Hi,

On 20/03/2019 16:21, Volodymyr Babchuk wrote:
> Julien Grall writes:
> > On 07/03/2019 21:04, Volodymyr Babchuk wrote:
> > > From: Volodymyr Babchuk <vlad.babchuk@xxxxxxxxx>
> > >
> > > OP-TEE usually uses the same idea with command buffers (see
> > > previous commit) to issue RPC requests. Problem is that initially
> > > it has no buffer, where it can write request. So the first RPC
> > > request it makes is special: it requests NW to allocate shared
> > > buffer for other RPC requests. Usually this buffer is allocated
> > > only once for every OP-TEE thread and it remains allocated all
> > > the time until guest shuts down. Guest can ask OP-TEE to disable
> > > RPC buffers caching, in this case OP-TEE will ask guest to
> > > allocate/free buffer for the each RPC.
> > >
> > > Mediator needs to pin this buffer to make sure that domain can't
> > > transfer it to someone else.
> > At the moment, Xen on Arm doesn't support transfer of a page between
> > domain (see steal_page). What we want to prevent here is the domain to
> > free the page (via XENMEM_decrease_reservation). If the reference drop
> > to 0, the page will be freed and could potentially be allocated for
> > Xen usage or another domain. Taking the reference here, will prevent
> > it to free until the reference is dropped.
> >
> > So I would reword this sentence. Something like:
> >
> > "Mediator needs to pin the buffer to make sure the page will not be
> > freed while it is shared with OP-TEE".
> When I wrote that, I kept in mind grant mappings as well. But, I
> think, I was mistaken there, because grant mappings will have p2m type
> p2m_grant_map_rw/p2m_grant_map_ro. So yeah, your variant is better. Thanks.
I don't understand how grant mappings are related to this patch. What did you 
have in mind about them?
Cheers,

--
Julien Grall

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