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

[Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>]

Hi,

On 12.09.18 14:02, Julien Grall wrote:
> 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(...).
Yes, this is what I was saying. 512 pages per call.

> > > 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.
Strictly speaking, yes. All steps has cost: trap to EL3, dispatch in EL3,
switch to S-EL1, new thread preparation in OP-TEE, context switch in 
OP-TEE.
I wanted to tell, that in my opinion, this is negligible in comparison 
with the actual call processing. But maybe, I'm wrong there.
--
Volodymyr Babchuk

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