Re: [RFC PATCH 01/12] dma-buf: Introduce dma_buf_get_pfn_unlocked() kAPI
From: Christian König
Date: Mon Jan 20 2025 - 07:14:57 EST
Am 17.01.25 um 15:42 schrieb Simona Vetter:
On Wed, Jan 15, 2025 at 11:06:53AM +0100, Christian König wrote:
[SNIP]
Anything missing?
Well as far as I can see this use case is not a good fit for the DMA-buf
interfaces in the first place. DMA-buf deals with devices and buffer
exchange.
What's necessary here instead is to give an importing VM full access on some
memory for their specific use case.
That full access includes CPU and DMA mappings, modifying caching
attributes, potentially setting encryption keys for specific ranges etc....
etc...
In other words we have a lot of things the importer here should be able to
do which we don't want most of the DMA-buf importers to do.
This proposal isn't about forcing existing exporters to allow importers to
do new stuff. That stays as-is, because it would break things.
It's about adding yet another interface to get at the underlying data, and
we have tons of those already. The only difference is that if we don't
butcher the design, we'll be able to implement all the existing dma-buf
interfaces on top of this new pfn interface, for some neat maximal
compatibility.
That sounds like you missed my concern.
When an exporter and an importer agree that they want to exchange PFNs
instead of DMA addresses then that is perfectly fine.
The problems start when you define the semantics how those PFNs, DMA
address, private bus addresses, whatever is echanged different to what
we have documented for DMA-buf.
This semantics is very well defined for DMA-buf now, because that is
really important or otherwise things usually seem to work under testing
(e.g. without memory pressure) and then totally fall apart in production
environments.
In other words we have defined what lock you need to hold when calling
functions, what a DMA fence is, when exchanged addresses are valid etc...
But fundamentally there's never been an expectation that you can take any
arbitrary dma-buf and pass it any arbitrary importer, and that is must
work. The fundamental promise is that if it _does_ work, then
- it's zero copy
- and fast, or as fast as we can make it
I don't see this any different than all the much more specific prposals
and existing code, where a subset of importers/exporters have special
rules so that e.g. gpu interconnect or vfio uuid based sharing works.
pfn-based sharing is just yet another flavor that exists to get the max
amount of speed out of interconnects.
Please take another look at what is proposed here. The function is
called dma_buf_get_pfn_*unlocked* !
This is not following DMA-buf semantics for exchanging addresses and
keeping them valid, but rather something more like userptrs.
Inserting PFNs into CPU (or probably also IOMMU) page tables have a
different semantics than what DMA-buf usually does, because as soon as
the address is written into the page table it is made public. So you
need some kind of mechanism to make sure that this addr you made public
stays valid as long as it is public.
The usual I/O operation we encapsulate with DMA-fences have a
fundamentally different semantics because we have the lock which
enforces that stuff stays valid and then have a DMA-fence which notes
how long the stuff needs to stay valid for an operation to complete.
Regards,
Christian.
Cheers, Sima
The semantics for things like pin vs revocable vs dynamic/moveable seems
similar, but that's basically it.
As far as I know the TEE subsystem also represents their allocations as file
descriptors. If I'm not completely mistaken this use case most likely fit's
better there.
I feel like this is small enough that m-l archives is good enough. For
some of the bigger projects we do in graphics we sometimes create entries
in our kerneldoc with wip design consensus and things like that. But
feels like overkill here.
My general desire is to move all of RDMA's MR process away from
scatterlist and work using only the new DMA API. This will save *huge*
amounts of memory in common workloads and be the basis for non-struct
page DMA support, including P2P.
Yeah a more memory efficient structure than the scatterlist would be
really nice. That would even benefit the very special dma-buf exporters
where you cannot get a pfn and only the dma_addr_t, altough most of those
(all maybe even?) have contig buffers, so your scatterlist has only one
entry. But it would definitely be nice from a design pov.
Completely agree on that part.
Scatterlist have a some design flaws, especially mixing the input and out
parameters of the DMA API into the same structure.
Additional to that DMA addresses are basically missing which bus they belong
to and details how the access should be made (e.g. snoop vs no-snoop
etc...).
Aside: A way to more efficiently create compressed scatterlists would be
neat too, because a lot of drivers hand-roll that and it's a bit brittle
and kinda silly to duplicate. With compressed I mean just a single entry
for a contig range, in practice thanks to huge pages/folios and allocators
trying to hand out contig ranges if there's plenty of memory that saves a
lot of memory too. But currently it's a bit a pain to construct these
efficiently, mostly it's just a two-pass approach and then trying to free
surplus memory or krealloc to fit. Also I don't have good ideas here, but
dma-api folks might have some from looking at too many things that create
scatterlists.
I mailed with Christoph about that a while back as well and we both agreed
that it would probably be a good idea to start defining a data structure to
better encapsulate DMA addresses.
It's just that nobody had time for that yet and/or I wasn't looped in in the
final discussion about it.
Regards,
Christian.
-Sima