Re: [PATCH 2/2] rust: scatterlist: honor the device's maximum segment size

["Gary Guo" <gary@garyguo.net>]

On Mon Sep 14, 2026 at 1:17 PM BST, Robin Murphy wrote:
> On 14/09/2026 11:22 am, Gary Guo wrote:
>> On Mon Sep 14, 2026 at 10:58 AM BST, Danilo Krummrich wrote:
>>> On Mon Sep 14, 2026 at 2:45 AM CEST, Alexandre Courbot wrote:
>>>> On Mon Sep 14, 2026 at 6:08 AM JST, Matteo Kloiber wrote:
>>>>> On Mon Sep 7, 2026 at 11:43 AM JST, Alexandre Courbot wrote:
>>>>>> It also means that without patch 1, nova-core would split the firmware
>>>>>> into hundreds of 64KB SG entries, which is not breaking but still
>>>>>> something we want to avoid. The correct fix is to make sure that
>>>>>> `dma_set_max_seg_size` is called by the driver, and while we are at it
>>>>>> we also want every driver to call `dma_set_mask_and_coherent`. Ideally
>>>>>> we would use the type system to make sure that both functions are called
>>>>>> before any DMA operation can take place (using a safe interface), but
>>>>>> I'm not quite sure yet how we can do this.
>>>>>
>>>>> This sounds sensible indeed. Should I open a thread regarding that on Zulip?
>>>>
>>>> Probably not necessary, the mailing-list has a larger audience and is
>>>> the right place for this. I expect people will jump in here with their
>>>> thoughts.
>>>
>>> The problem with those is not that they must strictly be called before
>>> allocating DMA memory, but they must not be called concurrently with other DMA
>>> operations, such as allocating DMA memory, as it would technically be a data
>>> race.
>> 
>> Do they really have to be called *before* allocating DMA memory, not do they
>> just need not be called *concurrent* to DMA memory allocation?
>
> Similar to DMA masks, the segment parameters should be set appropriately 
> before any dma_map_sg() operation. Plus since they likely influence 
> scatterlist geometry, that means typically they're also going to need to 
> be set before building the scatterlist to be mapped in the first place.

Right, in that case I think we have the following options:

1.  Have setters these being unsafe, with precondition that they must be done
    before creation of DMA mappings.

2.  Have a `dma_info()` method like Danilo mentioned. However, we cannot expose
    `&Device<Core<'_>>` inside such callbacks, because otherwise driver would be
    able create DMA mappings. This however is too restrictive to be the only way
    of setting DMA masks, because Nova needs to access the bar before setting the
    DMA masks.

3.  Use wrapper type instead of generics for typestate. Then, we can express the
    pattern of typestate transformation (generics cannot do this because we
    always place it behind a reference).

    For example, we can have `Device` to mean `Device<Normal>`, and then have
    `Bound<Device>` to mean `Device<Bound>` (both of which are behind a shared
    reference, like today, and `Bound<D>` can deref to `&D`).

    Then, for `Core` typestate, we can instead have

        struct Core<'a, D>(&'a Bound<Device>);

    where it's passed by value. We can then define the DMA setters methods to
    operate on `Core` like we did today, but safely. Instead of providing a
    `Deref` impl that turns `&Core<'_, D>` to `&Bound<D>`, we only implement
    forwarding functions for APIs that can work without DMA configured (e.g. PCI
    bar).

    Then, provide a `into_bound()` method which *consumes* `Core<'a, D>` and
    give out `&'a Bound<Device>` (this is why typestate generics cannot work,
    because `&'a Device<Core<'_>>` is Copy and we cannot represent the ownership
    consumption).

>
> I don't think there's any expectation that they would ever change 
> *between* different mappings - especially given the underlying hardware 
> properties they represent - so while that might technically be possible 
> in the C API if the caller can enforce sufficient ordering, it should be 
> fine to rule it out in the Rust abstraction if that makes things easier 
> to reason about.
>
> Thanks,
> Robin.
>
>> If it's the former, we can require these to require mutable reference instead,

Correction: this should say "latter".

Best,
Gary

>> so the probe takes `Pin<&'bound mut Device<Core<'_>>>` which still derefs to
>> `&'bound Device<Bound>`, but Rust will require the shared reference to not
>> co-exist with the mutable reference.
>> 
>> Best,
>> Gary
>>