After reading up on some internal discussion with Marek and some other
people I stumbled over another very important think to keep in mind:
We have tons of hardware specific stuff in the VM subsystem!
This ranges from specific permission flags (valid, executable, read,
write are probably common enough, but "fetchable" ?), over things like
cache control towards things like internal data routing flags.
If this should be device independent we absolute need at least a 64bit
hardware flags value in the interface and then obviously ways how device
independent code can figure out the flags to use for a specific use case.
Even better would be an additional pointer to a hardware driver private
data structure for the mapping.
Regards,
Christian.
Am 26.08.22 um 05:11 schrieb Brost, Matthew:
Sorry for the outlook reply, so in XE that works the opposite of
AMDGPU. Mappings keep a reference to the BO and the mapping exist
until they are explicitly destroyed or the VM is destroyed, so if a
mapping exists the BO exists. Quickly implemented a prototype
extension to the VM bind IOCTL that blows away all mappings on a BO
per Jason’s suggestion, for XE it was really straight forward.
I’d have to double check the i915 reference counting wrt to BO and
mappings but I suspect it works like XE.
IMO this paradigm is the way to go as it does match open / mmap /
close semantics.
Matt
*From:* Christian König <christian.koe...@amd.com>
*Sent:* Thursday, August 25, 2022 6:37 AM
*To:* Jason Ekstrand <ja...@jlekstrand.net>
*Cc:* Bas Nieuwenhuizen <b...@basnieuwenhuizen.nl>; Dave Airlie
<airl...@gmail.com>; dri-devel <dri-devel@lists.freedesktop.org>;
Daniel Vetter <daniel.vet...@ffwll.ch>; Brost, Matthew
<matthew.br...@intel.com>; Ben Skeggs <skeg...@gmail.com>
*Subject:* Re: vm binding interfaces and parallel with mmap
Am 24.08.22 um 18:14 schrieb Jason Ekstrand:
On Mon, Aug 22, 2022 at 8:27 AM Christian König
<christian.koe...@amd.com> wrote:
[SNIP]
>> I suppose it also asks the question around paralleling
>>
>> fd = open()
>> ptr = mmap(fd,)
>> close(fd)
>> the mapping is still valid.
>>
>> I suppose our equiv is
>> handle = bo_alloc()
>> gpu_addr = vm_bind(handle,)
>> gem_close(handle)
>> is the gpu_addr still valid does the VM hold a reference on
the kernel
>> bo internally.
> For Vulkan it looks like this is undefined and the above is
not necessary:
>
> "It is important to note that freeing a VkDeviceMemory
object with
> vkFreeMemory will not cause resources (or resource regions)
bound to
> the memory object to become unbound. Applications must not
access
> resources bound to memory that has been freed."
> (32.7.6)
I'm not sure about this particular question. We need to be sure
that maps get cleaned up eventually. On the one hand, I think
it's probably a valid API implementation to have each mapped page
hold a reference similar to mmap and have vkDestroyImage or
vkDestroyBuffer do an unmap to clean up the range. However,
clients may be surprised when they destroy a large memory object
and can't reap the memory because of extra BO references they
don't know about. If BOs unmap themselves on close or if we had a
way to take a VM+BO and say "unmap this BO from everywhere,
please", we can clean up the memory pretty easily. Without that,
it's a giant PITA to do entirely inside the userspace driver
because it requires us to globally track every mapping and that
means data structures and locks. Yes, such an ioctl would require
the kernel to track things but the kernel is already tracking
everything that's bound, so hopefully it doesn't add much.
For both amdgpu as well as the older radeon mapping a BO does *not*
grab a reference to it. Whenever a BO is released all it's mappings
just disappear.
We need to keep track of the mappings anyway to recreate page tables
after (for example) suspend and resume, so that isn't any overhead.
Regards,
Christian.
--Jason
Additional to what was discussed here so far we need an array
on in and
out drm_syncobj for both map as well as unmap.
E.g. when the mapping/unmapping should happen and when it is
completed
etc...
Christian.
>
>
>> Dave.
>>> Dave.
