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

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

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?

Cheers,

-- 
Julien Grall

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