Re: [PATCH v8 1/1] rust: pci: add extended capability and SR-IOV support

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

On Mon Aug 24, 2026 at 12:14 PM BST, Alexandre Courbot wrote:
> On Mon Aug 24, 2026 at 7:48 PM JST, Gary Guo wrote:
>> On Mon Aug 24, 2026 at 9:12 AM BST, Alexandre Courbot wrote:
>>> On Tue Aug 18, 2026 at 5:46 PM JST, Zhi Wang wrote:
>>>> Rust PCI drivers have no typed interface for locating and accessing PCIe
>>>> extended capabilities.
>>>>
>>>> The SR-IOV extended capability describes VF topology and VF BARs. Expose
>>>> this information through the Rust PCI abstraction so drivers can use the
>>>> existing typed configuration-space accessors instead of raw bindings.
>>>>
>>>> Define ExtCapability to associate a capability ID with a register layout,
>>>> and add ConfigSpace::find_ext_capability() to locate and project that
>>>> layout. Bound the view at the next capability or the end of extended
>>>> configuration space. Add ExtSriovRegs and a decoded VF BAR iterator that
>>>> reads and validates all six VF BAR register slots up front, yields decoded
>>>> BAR addresses and widths in logical order, and keeps the raw
>>>> configuration-space slot advancement internal. Since PCI_EXT_CAP_NEXT() is
>>>> a function-like macro, expose it through a Rust helper.
>>>>
>>>> Link: https://lore.kernel.org/rust-for-linux/20260804161612.776752-1-zhiw@nvidia.com/
>>>> Cc: Alexandre Courbot <acourbot@nvidia.com>
>>>> Cc: Gary Guo <gary@garyguo.net>
>>>> Signed-off-by: Zhi Wang <zhiw@nvidia.com>
>>>> ---
>>>>  rust/helpers/pci.c     |   5 +
>>>>  rust/kernel/pci.rs     |   8 +
>>>>  rust/kernel/pci/cap.rs | 329 +++++++++++++++++++++++++++++++++++++++++
>>>>  3 files changed, 342 insertions(+)
>>>>  create mode 100644 rust/kernel/pci/cap.rs
>>>>
>>>> diff --git a/rust/helpers/pci.c b/rust/helpers/pci.c
>>>> index 4ebf256dff23..b946b14d79e4 100644
>>>> --- a/rust/helpers/pci.c
>>>> +++ b/rust/helpers/pci.c
>>>> @@ -24,6 +24,11 @@ __rust_helper bool rust_helper_dev_is_pci(const struct device *dev)
>>>>  	return dev_is_pci(dev);
>>>>  }
>>>>  
>>>> +__rust_helper u32 rust_helper_pci_ext_cap_next(u32 header)
>>>> +{
>>>> +	return PCI_EXT_CAP_NEXT(header);
>>>> +}
>>>> +
>>>>  #ifndef CONFIG_PCI_IOV
>>>>  __rust_helper unsigned int
>>>>  rust_helper_pci_sriov_get_totalvfs(struct pci_dev *pdev)
>>>> diff --git a/rust/kernel/pci.rs b/rust/kernel/pci.rs
>>>> index 9f19ccd5905c..008c2770a3f3 100644
>>>> --- a/rust/kernel/pci.rs
>>>> +++ b/rust/kernel/pci.rs
>>>> @@ -32,10 +32,18 @@
>>>>      },
>>>>  };
>>>>  
>>>> +mod cap;
>>>>  mod id;
>>>>  mod io;
>>>>  mod irq;
>>>>  
>>>> +pub use self::cap::{
>>>> +    ExtCapId,
>>>> +    ExtCapability,
>>>> +    ExtSriovCapability,
>>>> +    ExtSriovRegs,
>>>> +    ExtSriovVfBar, //
>>>> +};
>>>>  pub use self::id::{
>>>>      Class,
>>>>      ClassMask,
>>>> diff --git a/rust/kernel/pci/cap.rs b/rust/kernel/pci/cap.rs
>>>> new file mode 100644
>>>> index 000000000000..ddb3fd73e195
>>>> --- /dev/null
>>>> +++ b/rust/kernel/pci/cap.rs
>>>> @@ -0,0 +1,329 @@
>>>> +// SPDX-License-Identifier: GPL-2.0
>>>> +
>>>> +//! PCI extended capability support.
>>>> +
>>>> +use super::{
>>>> +    io::ConfigSpaceBackend,
>>>> +    ConfigSpace,
>>>> +    Extended, //
>>>> +};
>>>
>>> Let's merge this block with the one below, i.e. using `crate::pci`?
>>>
>>>> +use crate::{
>>>> +    bindings,
>>>> +    io::{
>>>> +        Io,
>>>> +        IoBackend,
>>>> +        Region, //
>>>> +    },
>>>> +    num::Bounded,
>>>> +    prelude::*,
>>>> +};
>>>> +
>>>> +/// Number of VF BAR register slots in an SR-IOV capability.
>>>> +// CAST: `PCI_SRIOV_NUM_BARS` is the PCIe-specified number of VF BAR register slots and fits in
>>>> +// `usize`.
>>>> +const NUM_VF_BARS: usize = bindings::PCI_SRIOV_NUM_BARS as usize;
>>>
>>> The infallible casts module is now available in `master`. If you import
>>> `crate::num::casts` you can now turn this into
>>>
>>>     const NUM_VF_BARS: usize = casts::u32_as_usize(bindings::PCI_SRIOV_NUM_BARS);
>>>
>>> and remove the `CAST` comment.
>>>
>>>> +
>>>> +/// PCI extended capability IDs.
>>>> +#[repr(transparent)]
>>>> +#[derive(Debug, Clone, Copy, PartialEq, Eq)]
>>>> +pub struct ExtCapId(u16);
>>>> +
>>>> +impl ExtCapId {
>>>> +    /// Single Root I/O Virtualization.
>>>> +    // CAST: PCI extended capability IDs are 16-bit values defined by the PCIe specification.
>>>> +    pub const SRIOV: Self = Self(bindings::PCI_EXT_CAP_ID_SRIOV as u16);
>>>
>>> Same here, the `CAST` comment can be removed if you turn this line into
>>>
>>>     pub const SRIOV: Self = Self(casts::u32_into_u16::<{ bindings::PCI_EXT_CAP_ID_SRIOV }>());
>>
>> This looks horrible. I'd prefer `as`.
>
> Looks are subjective (or so I like to tell myself), so let's focus on
> what isn't.
>
> What is not subjective is that you don't lose a single bit as long as
> you use these functions, and can do your conversions without a `CAST`
> comment.

Aesthetic and ergnomics is a big part of code, and is what make people like
Rust. In a world where less code is produced by human, I think it's especially
important that code remains easily human readable.

You can probably already tell that I have very strong opinion about this.

>
> These helpers were merged precisely for this use-case; if you want to
> remove them and use `as` instead, that's of course open but also a
> larger discussion than this patch.

I was under impression that the series just provides safe cast from lower width
integer to higher width ones (mostly for u32 -> usize and usize -> u64). If I
see the above snippet, I'd nacked it.

This is not what const generics is designed for and I rather view this as abuse
of language feature.

Best,
Gary