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

From: Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>
To: Julien Grall <julien.grall@arm.com>
Cc: "tee-dev@lists.linaro.org" <tee-dev@lists.linaro.org>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	Stefano Stabellini <sstabellini@kernel.org>,
	Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>
Subject: Re: [Xen-devel] [PATCH 1/5] xen/arm: optee: impose limit on shared buffer size
Date: Mon, 16 Sep 2019 15:26:53 +0000	[thread overview]
Message-ID: <87d0g0feir.fsf@epam.com> (raw)
In-Reply-To: <24f24620-99c9-d266-5691-4e1962d67b27@arm.com>

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.

-- 
Volodymyr Babchuk at EPAM
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