On 11/7/2023 5:22 PM, Gaurav Kohli wrote:
On 10/24/2023 2:32 AM, Rob Clark wrote:
On Wed, Sep 20, 2023 at 6:12 AM Steven Price <steven.price@xxxxxxx>
wrote:
On 09/08/2023 13:17, Boris Brezillon wrote:
Hello,
This patchset is an attempt at making page table allocation
customizable. This is useful to some GPU drivers for various reasons:
- speed-up upcoming page table allocations by managing a pool of free
pages
- batch page table allocation instead of allocating one page at a time
- pre-reserve pages for page tables needed for map/unmap operations and
return the unused page tables to some pool
The first and last reasons are particularly important for GPU drivers
wanting to implement asynchronous VM_BIND. Asynchronous VM_BIND
requires
that any page table needed for a map/unmap operation to succeed be
allocated at VM_BIND job creation time. At the time of the job
creation,
we don't know what the VM will look like when we get to execute the
map/unmap, and can't guess how many page tables we will need. Because
of that, we have to over-provision page tables for the worst case
scenario (page table tree is empty), which means we will allocate/free
a lot. Having pool a pool of free pages is crucial if we want to
speed-up VM_BIND requests.
A real example of how such custom allocators can be used is available
here[1]. v2 of the Panthor driver is approaching submission, and I
figured I'd try to upstream the dependencies separately, which is
why I submit this series now, even though the user of this new API
will come afterwards. If you'd prefer to have those patches submitted
along with the Panthor driver, let me know.
This approach has been discussed with Robin, and is hopefully not too
far from what he had in mind.
The alternative would be to embed a cache of pages into the IOMMU
framework, however kmem_cache sadly doesn't seem to support the
'reserve' of pages concept that we need. mempools could be a solution
but the mempool would need to be created by the IOMMU framework as the
alloc/free functions are specified when creating the pool. So it would
be a much bigger change (to drivers/iommu).
So, given that so far it's just Panthor this seems like the right
approach for now - when/if other drivers want the same functionality
then it might make sense to revisit the idea of doing the caching within
the IOMMU framework.
I have some plans to use this as well for drm/msm.. but the reasons
and requirements are basically the same as for panthor. I think I
prefer the custom allocator approach, rather than tying this to IOMMU
framework. (But ofc custom allocators, I guess, does not prevent the
iommu driver from doing it's own caching.)
BR,
-R
We have also posted one RFC patch series which is based on this current
patches by Boris and helping us to define our custom alloc and free
pgtable call. For our side usecase we have a requirement to create
pgtable from HLOS and then share it to different entity(VMID) and
basically that also requires few smc calls and for that we need
custom alloc/free callbacks.
https://lore.kernel.org/all/20231101071144.16309-1-quic_gkohli@xxxxxxxxxxx/
So custom allocator and free ops is helping for us also. Is there any
plan to merge these patches from Boris.
Please use below tested by tag, if you are planning to merge patches by
Boris:
Tested-by: Gaurav Kohli <quic_gkohli@xxxxxxxxxxx>
Robin: Does this approach sound sensible?
FWIW:
Reviewed-by: Steven Price <steven.price@xxxxxxx>
Steve
Regards,
Boris
[1]https://gitlab.freedesktop.org/panfrost/linux/-/blob/panthor/drivers/gpu/drm/panthor/panthor_mmu.c#L441
Boris Brezillon (2):
iommu: Allow passing custom allocators to pgtable drivers
iommu: Extend LPAE page table format to support custom allocators
drivers/iommu/io-pgtable-arm.c | 50
+++++++++++++++++++++++-----------
drivers/iommu/io-pgtable.c | 31 +++++++++++++++++++++
include/linux/io-pgtable.h | 21 ++++++++++++++
3 files changed, 86 insertions(+), 16 deletions(-)
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