Re: [RFC PATCH V2 13/22] x86/intel_rdt: Support schemata write - pseudo-locking core

[Reinette Chatre <reinette.chatre@intel.com>]

Hi Mike,

On 2/20/2018 5:58 PM, Mike Kravetz wrote:
> On 02/20/2018 03:21 PM, Thomas Gleixner wrote:
>> On Tue, 20 Feb 2018, Reinette Chatre wrote:
>>> On 2/20/2018 9:15 AM, Thomas Gleixner wrote:
>>>> On Tue, 13 Feb 2018, Reinette Chatre wrote:
>>>>
>>>> Now the remaining thing is the memory allocation and the mmap itself. I
>>>> really dislike the preallocation of memory right at setup time. Ideally
>>>> that should be an allocation of the application itself, but the horrid
>>>> wbinvd stuff kinda prevents that. With that restriction we are more or less
>>>> bound to immediate allocation and population.
>>>
>>> Acknowledged. I am not sure if the current permissions would support
>>> such a dynamic setup though. At this time the system administrator is
>>> the one that sets up the pseudo-locked region and can through
>>> permissions of the character device provide access to these regions to
>>> user space applications.
>>
>> You still would need some interface, e.g. character device which allows you
>> to hand in the pointer to the user allocated memory and do the cache
>> priming. So you could use the same permission setup for that character
>> device.
>>
>> The other problem is that we'd need to have MAP_CONTIG first so you
>> actually can allocate physically contigous memory from user space. Mike is
>> working on that, but it's not available today. The only way to do so today
>> (with lots of waste) would be MAP_HUGETLB, which might be an acceptable
>> constraint up to the point where MAP_CONTIG is available.
> 
> Just to clarify, there is not any activity on exposing a general purpose
> MAP_CONTIG interface to user space.  When initially proposed, MAP_CONTIG
> was shot down and the suggestion was to create a new in kernel interface
> to make allocation of contiguous pages easier.  The initial use case was
> a driver which could use the new internal interface as part of it's
> mmap() routine to give contiguous regions to user space.
> 
> Reinette is using this new interface, but that must be for the ?immediate
> allocation? case you are trying to move away from.  Sorry, I have not been
> following development of this feature.
> 
> If you would have to create a device to accept a user buffer, could you
> perhaps use the same device to create/hand out a contiguous mapping?

Thank you very much for keeping an eye on this discussion. I do still
intend to implement the immediate allocation case by using the new
find_alloc_contig_pages()/free_contig_pages().

Reinette