Re: GEM allocation for para-virtualized DRM driver

[Oleksandr Andrushchenko <andr2000@gmail.com>]

On 03/18/2017 04:06 PM, Rob Clark wrote:
> On Sat, Mar 18, 2017 at 9:25 AM, Oleksandr Andrushchenko
> <andr2000@gmail.com> wrote:
>> Hi, Rob
>>
>> On 03/18/2017 02:22 PM, Rob Clark wrote:
>>> On Fri, Mar 17, 2017 at 1:39 PM, Oleksandr Andrushchenko
>>> <andr2000@gmail.com> wrote:
>>>> Hello,
>>>> I am writing a para-virtualized DRM driver for Xen hypervisor
>>>> and it now works with DRM CMA helpers, but I would also like
>>>> to make it work with non-contigous memory: virtual machine
>>>> that the driver runs in can't guarantee that CMA is actually
>>>> physically contigous (that is not a problem because of IPMMU
>>>> and other means, the only constraint I have is that I cannot mmap
>>>> with pgprot == noncached). So, I am planning to use *drm_gem_get_pages* +
>>>> *shmem_read_mapping_page_gfp* to allocate memory for GEM objects
>>>> (scanout buffers + dma-bufs shared with virtual GPU)
>>>>
>>>> Do you think this is the right approach to take?
>>> I guess if you had some case where you needed to "migrate" buffers
>>> between host and guest memory,
>> yes, this is the case. but, I can "map" buffers between host and guests
> if you need to physically copy (transfer), like a discreet gpu with
> vram, then TTM makes sense.  If you can map the pages directly into
> the guest then TTM is probably overkill.
We have zero copy from guest to host/HW, this is why I'm not considering TTM
>>>    then TTM might be useful.
>> I was looking into it, but it seems to be an overkill in my case
>> And isn't it that GEM should be used for new drivers, not TTM?
> Not really, it's just that (other than amdgpu which uses TTM) all of
> the newer drivers have been unified memory.
Good to know, thank you
>    A driver for a new GPU
> that had vram of some sort should still use TTM.
our virtual GPU support is done on hypervisor level, so no changes to
existing GPU drivers. So, the only thing to care about is that the
buffers our DRM driver provides can be imported and used by that GPU
(there are other issues related to memory, e.g. if real GPU/firware can
see the memory of the guest, but this is another story)
> BR,
> -R
>
>>>     Otherwise
>>> this sounds like the right approach.
>> Thank you. Actually, I am playing with alloc_pages + remap_pfn_range now,
>> but what DRM provides (_get_pages + shmem_read) seem to be more portable
>> and generic. So, I'll probably stick to it
>>> BR,
>>> -R
>> Thank you for helping,
>> Oleksandr Andrushchenko
Ok, then I'll drop my alloc_pages + remap_pfn_range in favor of
drm_gem_get_pages + shmem_read_mapping_page_gfp

Thank you
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