Re: [PATCH v4 09/20] rust: io: use view types instead of addresses for `Io`

[Gary Guo]

On Sun Jun 21, 2026 at 5:15 PM BST, Danilo Krummrich wrote:
> On Sun Jun 21, 2026 at 5:41 PM CEST, Gary Guo wrote:
>> On Sun Jun 21, 2026 at 10:17 AM BST, Alexandre Courbot wrote:
>>> As you can see, this lets a 32-bit access be done on the upper half of a
>>> 64-bit register, which sounds like it should not be allowed? Similarly one
>>> could change register types, and so on. This might not be "unsafe" in the
>>> sense that it is still aligned and in bounds, but it lets the structure set
>>> by the type system be bypassed. It could also potentially be a violation of
>>> the hardware contract if the access width is relevant for this particular
>>> address.
>>
>> I see no reason to prevent any of the case, this can be done by `try_cast()`
>> API as well. If we need to take access width restriction and other
>> restrictions into consideration, then a lot of API cannot be exposed at all.
>> E.g. it is not okay to add `copy_read`/`copy_write` like the patch 19, because
>> it uses memcpy_from/toio which is possibility doing byte-width access.
>>
>> in my opinion think people should be able to type casting without reaching out
>> to `unsafe` if it's not UB. Similar to the logic on why we have `zerocopy`
>> that allows casting between to types, these are "bypassing the typesystem" as
>> well!
>
> I think this is fine as-is. The natural, ergonomic path through the API
> (io_read!/io_write!/io_project! macros, IoLoc-based accessors, etc.) leads users
> toward correct access widths.
>
> Whether through io_addr() or a custom IoLoc implementation, reinterpreting the
> access type requires explicit, deliberate choices: picking a different type and
> computing a byte offset. This is not something anyone would do by accident.
>
> So, as long as the API doesn't provide a subtle way to do the wrong thing by
> accident, I don't think we need to add restrictions here.

I'm still going to make this a standalone function to be used from `IoLoc`
functions only, given these functions are not intended to be used by user
directly anyway. But I'll leave `IoLoc` and `try_cast` as-is.

Best,
Gary