Re: [PATCH RFC 18/18] drm/asahi: Add the Asahi driver for Apple AGX GPUs

[Asahi Lina <lina@asahilina.net>]

On 05/04/2023 23.37, Daniel Vetter wrote:
> On Tue, Mar 07, 2023 at 11:25:43PM +0900, Asahi Lina wrote:
>> +/// A generic monotonically incrementing ID used to uniquely identify object instances within the
>> +/// driver.
>> +pub(crate) struct ID(AtomicU64);
>> +
>> +impl ID {
>> +    /// Create a new ID counter with a given value.
>> +    fn new(val: u64) -> ID {
>> +        ID(AtomicU64::new(val))
>> +    }
>> +
>> +    /// Fetch the next unique ID.
>> +    pub(crate) fn next(&self) -> u64 {
>> +        self.0.fetch_add(1, Ordering::Relaxed)
>> +    }
>> +}
> 
> Continuing the theme of me commenting on individual things, I stumbled
> over this because I noticed that there's a lot of id based lookups where I
> don't expect them, and started chasing.
> 
> - For ids use xarray, not atomic counters. Yes I know dma_fence timelines
>    gets this wrong, this goes back to an innocent time where we didn't
>    allocate more than one timeline per engine, and no one fixed it since
>    then. Yes u64 should be big enough for everyone :-/
> 
> - Attaching ID spaces to drm_device is also not great. drm is full of
>    these mistakes. Much better if their per drm_file and so private to each
>    client.
> 
> - They shouldn't be used for anything else than uapi id -> kernel object
>    lookup at the beginning of ioctl code, and nowhere else. At least from
>    skimming it seems like these are used all over the driver codebase,
>    which does freak me out. At least on the C side that's a clear indicator
>    for a refcount/lockin/data structure model that's not thought out at
>    all.
> 
> What's going on here, what do I miss?

These aren't UAPI IDs, they are driver-internal IDs (the UAPI IDs do use 
xarray and are per-File). Most of them are just for debugging, so that 
when I enable full debug spam I have some way to correlate different 
things that are happening together (this subset of interleaved log lines 
relate to the same submission). Basically just object names that are 
easier to read (and less of a security leak) than pointers and 
guaranteed not to repeat. You could get rid of most of them and it 
wouldn't affect the driver design, it just makes it very hard to see 
what's going on with debug logs ^^;

There are only two that are ever used for non-debugging purposes: the VM 
ID, and the File ID. Both are per-device global IDs attached to the VMs 
(not the UAPI VM objects, but rather the underlyng MMU address space 
managers they represent, including the kernel-internal ones) and to 
Files themselves. They are used for destroying GEM objects: since the 
objects are also device-global across multiple clients, I need a way to 
do things like "clean up all mappings for this File" or "clean up all 
mappings for this VM". There's an annoying circular reference between 
GEM objects and their mappings, which is why this is explicitly coded 
out in destroy paths instead of naturally happening via Drop semantics 
(without that cleanup code, the circular reference leaks it).

So e.g. when a File does a GEM close or explicitly asks for all mappings 
of an object to be removed, it goes out to the (possibly shared) GEM 
object and tells it to drop all mappings marked as owned by that unique 
File ID. When an explicit "unmap all in VM" op happens, it asks the GEM 
object to drop all mappings for that underlying VM ID. Similarly, when a 
UAPI VM object is dropped (in the Drop impl, so both explicitly and when 
the whole File/xarray is dropped and such), that does an explicit unmap 
of a special dummy object it owns which would otherwise leak since it is 
not tracked as a GEM object owned by that File and therefore not handled 
by GEM closing. And again along the same lines, the allocators in 
alloc.rs explicitly destroy the mappings for their backing GEM objects 
on Drop. All this is due to that annoying circular reference between VMs 
and GEM objects that I'm not sure how to fix.

Note that if I *don't* do this (or forget to do it somewhere) the 
consequence is just that we leak memory, and if you try to destroy the 
wrong IDs somehow the worst that can happen is you unmap things you 
shouldn't and fault the GPU (or, in the kernel or kernel-managed user VM 
cases, potentially the firmware). Rust safety guarantees still keep 
things from going entirely off the rails within the kernel, since 
everything that matters is reference counted (which is why these 
reference cycles are possible at all).

This all started when I was looking at the panfrost driver for 
reference. It does the same thing except it uses actual pointers to the 
owning entities instead of IDs, and pointer comparison (see 
panfrost_gem_close). Of course you could try do that in Rust too 
(literally storing and comparing raw pointers that aren't owned 
references), but then you're introducing a Pin<> requirement on those 
objects to make their addresses stable and it feels way more icky and 
error-prone than unique IDs (since addresses can be reused). panfrost 
only has a single mmu (what I call the raw VM) per File while I have an 
arbitrary number, which is why I end up with the extra 
distinction/complexity of both File and VM IDs, but the concept is the same.

Some of this is going to be refactored when I implement arbitrary VM 
range mapping/unmapping, which would be a good time to improve this... 
but is there something particularly wrong/broken about the way I'm doing 
it now that I missed? I figured unique u64 IDs would be a pretty safe 
way to identify entities and cleanup the mappings when needed.

~~ Lina