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. 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? 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. -- Volodymyr Babchuk at EPAM _______________________________________________ Xen-devel mailing list Xen-devel@lists.xenproject.org https://lists.xenproject.org/mailman/listinfo/xen-devel
