linux-kernel - Re: [PATCH 2/3 v6] ACPI: allow longer device IDs

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Message-ID: <CAHp75VcjrD3kwN1BfWpjKXaVpG7MHfftMUscSGhcJfStm4b-Xg@mail.gmail.com>
Date:   Mon, 28 Feb 2022 22:46:07 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-crypto <linux-crypto@...r.kernel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Alexander Graf <graf@...zon.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Hans de Goede <hdegoede@...hat.com>,
        Len Brown <lenb@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH 2/3 v6] ACPI: allow longer device IDs

On Mon, Feb 28, 2022 at 9:28 PM Jason A. Donenfeld <Jason@...c4.com> wrote:
>
> From: Alexander Graf <graf@...zon.com>
>
> We create a list of ACPI "PNP" IDs which contains _HID, _CID, and CLS
> entries of the respective devices. However, when making structs for
> matching, we squeeze those IDs into acpi_device_id, which only has 9
> bytes space to store the identifier. The subsystem actually captures the
> full length of the IDs, and the modalias has the full length, but this
> struct we use for matching is limited. 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.

> 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.

Then why do we not see the ECR from somebody to update the spec or to
fix MS' abuse of it?
I believe _this_ should be the prerequisite to the proposed change.

> 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 on 64-bit, because the ulong in
> the same struct caused there to be 7 bytes of padding anyway, and when
> doing a s/M/Y/g i386_defconfig build, the bzImage only increased by
> 0.0055%, so negligible.
>
> 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.


-- 
With Best Regards,
Andy Shevchenko