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

[Julien Grall <julien.grall@xxxxxxx>]

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)
> > > > 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?

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.
Cheers,

--
Julien Grall

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