Re: [Xen-devel] [PATCH v2 09/13] optee: add support for arbitrary shared memory

[Julien Grall <julien.grall@xxxxxxx>]

On 09/11/2018 08:33 PM, Volodymyr Babchuk wrote:
> Hi Julien,
Hi,

> On 11.09.18 16:37, Julien Grall wrote:
> > Hi Volodymyr,
> >
> > On 10/09/18 19:04, Volodymyr Babchuk wrote:
> > > On 10.09.18 17:02, Julien Grall wrote:
> > > > On 03/09/18 17:54, Volodymyr Babchuk wrote:
> [...]
> > > > > +    if ( !pages_data_xen_start )
> > > > > +        return false;
> > > > > +
> > > > > +    shm_buf = allocate_shm_buf(ctx, param->u.tmem.shm_ref, 
> > > > > num_pages);
> > > > In alocate_shm_buf you are now globally limiting the number of pages 
> > > > ( (16384) to pin. However, this does not limit per call.
> > > > With the current limit, you would could call up to 16384 times 
> > > > lookup_and_pin_guest_ram_addr(...). On Arm, for p2m related 
> > > > operation, we limit to 512 iterations in one go before checking the 
> > > > preemption.
> > > > So I think 16384 times is far too much.
> > > So, in other words, I can translate only 2MB buffer (if 4096KB pages 
> > > are used), is it right?
> > 2MB for the whole command. So if you have 5 buffer in the command, 
> > then the sum of the buffer should not be bigger than 2MB.
> 4  buffers, but yes, it can be up to 8MB. Okay, I'll add per-call 
> counter to limit memory usage for a whole call.
That would need to be reduced to 2MB in total per call. You probably 
want to look at max_order(...).
> > However, 2MB might be too big considering that you also need to 
> > account the SMC call. Does buffer can be passed for fast call?
> No, all such calls are yielding calls, so you can ignore time used for 
> SMC call itself.
How come you can ignore it? It has a cost to trap to EL3.

Cheers,

--
Julien Grall

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