Re: [PATCH v5 2/3] ACPI: allow longer device IDs

Ard Biesheuvel
2022-02-27T07:32:11Z

On Sat, 26 Feb 2022 at 23:07, Jason A. Donenfeld <Jason@zx2c4.com> wrote:
>
> From: Alexander Graf <graf@amazon.com>
>

Please don't invent patch authors like that. Alex's patch that started
this discussion was completely different.

> We create a list of ACPI "PNP" IDs which contains _HID, _CID, and CLS
> entries of the respective devices. However, we squeeze them into struct
> acpi_device_id, which only has 9 bytes space to store the identifier. It
> originally had 16 bytes, but was changed to only have 9 in 6543becf26ff
> ("mod/file2alias: make modalias generation safe for cross compiling"),
> presumably on the theory that it would match the ACPI spec so it didn't
> matter.
>

Please clarify that this applies to the module metadata side of
things. The ACPI subsystem already captures and exposes _HIDs and
_CIDs that are longer than 8 characters, which is why simply
increasing the size of this field is sufficient to create modules that
can match devices that expose a CID that is longer than 8 bytes.

> Unfortunately, while most people adhere to the ACPI specs, Microsoft
> decided that its VM Generation Counter device [1] should only be
> identifiable by _CID with a value of "VM_Gen_Counter", which is longer
> than 9 characters.
>
> To allow device drivers to match identifiers that exceed the 9 byte
> limit, this simply ups the length to 16, just like it was before the
> aforementioned commit. Empirical testing indicates that this
> doesn't actually increase vmlinux size, because the ulong in the same
> struct caused there to be 7 bytes of padding anyway.
>

The padding situation only applies to struct acpi_device_id, whereas
ACPI_ID_LEN is used in other places as well. Also, the size of vmlinux
only covers statically allocated instances in the core kernel, and
most of the ACPI_ID_LEN uses are probably in drivers. So whether
vmlinux changes size or not is not that relevant.

> This patch is a prerequisite to add support for VMGenID in Linux, the
> subsequent patch in this series. It has been confirmed to also work on
> the udev/modalias side in userspace.
>
> [1] https://download.microsoft.com/download/3/1/C/31CFC307-98CA-4CA5-914C-D9772691E214/VirtualMachineGenerationID.docx
>
> Cc: Ard Biesheuvel <ardb@kernel.org>
> Cc: Len Brown <lenb@kernel.org>
> Cc: Rafael J. Wysocki <rafael@kernel.org>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Co-authored-by: Jason A. Donenfeld <Jason@zx2c4.com>
> Signed-off-by: Alexander Graf <graf@amazon.com>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

Patch 6543becf26ff was wrong to change ACPI_ID_LEN, because it failed
to take into account any other uses of ACPI_ID_LEN, and did not bother
to explain why the change was necessary in the context of what it was
trying to achieve.

So, given that we need more than 8 characters to match drivers to
devices exposed by Hyper-V (or other VMMs adhering to the VMGENID
spec), I think this change is necessary and correct.

So, with the authorship/signoff corrected, and the commit log clarified,

Reviewed-by: Ard Biesheuvel <ardb@kernel.org>

> ---
> Hi Rafael & Len,
>
> This patchset is directed toward you two specifically. Patches 1/3 and
> 3/3 have been through the ringer of review a bit already and do not
> specifically require your attention, but in v4 we wound up getting hung
> up on an ACPI API limitation. This v5 fixes that limitation with this
> 2/3 patch that you see here, with a trivial one line fix, which does
> require your attention.
>
> Patches 1/3 and 3/3 will go through my random.git tree. However, 3/3
> actually depends on this one here, 2/3, in order to compile without
> warnings (and be functional at all). Therefore, it would be nice if you
> would provide an "Acked-by" on it and permit me to /also/ take it
> through my random.git tree (if it looks like a correct patch to you, of
> course). This would make the merge logistics a lot easier. Plus it's a
> small +1/-1 line change.
>
> Please have a look and let me know what you think.
>
> Thanks,
> Jason
>
> include/linux/mod_devicetable.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/mod_devicetable.h b/include/linux/mod_devicetable.h
> index 4bb71979a8fd..5da5d990ff58 100644
> --- a/include/linux/mod_devicetable.h
> +++ b/include/linux/mod_devicetable.h
> @@ -211,7 +211,7 @@ struct css_device_id {
> kernel_ulong_t driver_data;
> };
>
> -#define ACPI_ID_LEN 9
> +#define ACPI_ID_LEN 16
>
> struct acpi_device_id {
> __u8 id[ACPI_ID_LEN];
> --
> 2.35.1
>