Re: [Xen-devel] [PATCH 1/5] xen/arm: optee: impose limit on shared buffer size

[Julien Grall <julien.grall@arm.com>]

Hi,

On 9/17/19 1:28 PM, Volodymyr Babchuk wrote:
> 
> Hi Julien,
> 
> 
> Julien Grall writes:
> 
>> Hi Volodymyr,
>>
>> On 9/16/19 4:26 PM, Volodymyr Babchuk wrote:
>>>
>>> Hi Julien,
>>>
>>> Julien Grall writes:
>>>
>>>> Hi,
>>>>
>>>> On 9/12/19 8:45 PM, Volodymyr Babchuk wrote:
>>>>>
>>>>> Hi Julien,
>>>>>
>>>>> Julien Grall writes:
>>>>>
>>>>>> Hi Volodymyr,
>>>>>>
>>>>>> On 9/11/19 7:48 PM, Volodymyr Babchuk wrote:
>>>>>>>
>>>>>>> Julien Grall writes:
>>>>>>>
>>>>>>>> Hi Volodymyr,
>>>>>>>>
>>>>>>>> On 8/23/19 7:48 PM, Volodymyr Babchuk wrote:
>>>>>>>>> We want to limit number of calls to lookup_and_pin_guest_ram_addr()
>>>>>>>>> per one request. There are two ways to do this: either preempt
>>>>>>>>> translate_noncontig() or to limit size of one shared buffer size.
>>>>>>>>>
>>>>>>>>> It is quite hard to preempt translate_noncontig(), because it is deep
>>>>>>>>> nested. So we chose second option. We will allow 512 pages per one
>>>>>>>>> shared buffer. This does not interfere with GP standard, as it
>>>>>>>>> requires that size limit for shared buffer should be at lest 512kB.
>>>>>>>>
>>>>>>>> Do you mean "least" instead of "lest"?
>>>>>>> Yes
>>>>>>>
>>>>>>>> If so, why 512 pages (i.e 1MB)
>>> I have missed that earlier. But 512 pages is 2MB, actually.
>>>
>>>>>>>> is plenty enough for most of the use cases? What does "xtest" consist
>>>>>>>> on?
>>>>>>> Bigger buffer xtest tries to allocate is mere 32KB. I believe that 1MB
>>>>>>> is enough for the most cases, because OP-TEE itself have a very limited
>>>>>>> resources. But this value is chosen arbitrary.
>>>>>>
>>>>>> Could we potentially reduce to let say 512KB (or maybe lower) if xtest
>>>>>> only allocate 32KB?
>>>>> Potentially - yes. But only to 512KB if we want to be compatible with
>>>>> the Global Platform specification. Why are you asking, though?
>>>>
>>>> Does the Global Platform specification limit to 512KB? Or is it a minimum?
>>> GP Spec says, that platform should allow *at lest* 512KB. Upper limit is
>>> not set.
>>>
>>>> Because, the smaller the buffer is, the less time it will take to
>>>> process in the worst case. Also, if we can have a reason for the size
>>>> (you seem to suggest the spec define a size...) then it is much better
>>>> than a random value.
>>> I have no strong arguments here, but I want to allow the biggest size
>>> possible. It seems, that 512 pages is the accepted limit in hypervisor
>>> code (at least, in p2m.c), so I chose this value.
>>
>> If GP specific request at least 512KB, then any portable code should
>> be able to deal with 512KB, right? So why would you allow more? What
>> are the cons/pros?
> Yes, any portable code should work with 512KB. I want to allow bigger
> buffers for two reasons: on OP-TEE issues tracking people often ask how
> to increase various memory limits across OP-TEE. So, apparently people
> sometimes wants bigger buffers. Second reasons is the non-portable
> things like Secure Data Path. For SDP one wants to pass media (like
> audio and video) data to and from OP-TEE. Media requires big buffers.

But what's the limit in that case? Would 2MB really be sufficient for them?

> 
> Anyways, I can see that 512 pages are established limit in the p2m
> code. So, why do you want OP-TEE mediator to have smaller limit?

One limit doesn't rule them all. ;)

The preemption is slightly more elaborate than checking the preemption 
every 512 pages. For instance in p2m_cache_flush_range(), we take into 
account how much is done.

In your case, you also have to factor how long the smc call is going to 
take in your calculation. This is not an exact science, but the fact 
that at the moment the limit for OP-TEE is much lower lead to me to 
think a smaller limit is better. Most likely OP-TEE will deny it anyway...

> 
> I want to be straight there: 512KB will likely work for most of the
> users. But there are always users who want more. So I would like to set
> largest plausible limit just in case.
I am pretty unconvinced that an higher value than what the spec request 
is right. But I don't have more ground than my gut feeling (I am always 
on the safe side). You are the maintainer of that code, so I am not 
going to push more for a lower value.

However, we should at least document any reasoning because this does not 
seem to be a that random value anymore.

Cheers,

-- 
Julien Grall

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