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

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Date:   Mon, 28 Feb 2022 23:21:54 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     "Michael Kelley (LINUX)" <mikelley@...rosoft.com>
Cc:     jason <jason@...c4.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        "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>,
        Hans de Goede <hdegoede@...hat.com>,
        Len Brown <lenb@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH 2/3 v6] ACPI: allow longer device IDs

On Mon, 28 Feb 2022 at 23:14, Michael Kelley (LINUX)
<mikelley@...rosoft.com> wrote:
>
> From: Jason A. Donenfeld <Jason@...c4.com> Sent: Monday, February 28, 2022 1:55 PM
> >
> > Hi Andy,
> >
> > On Mon, Feb 28, 2022 at 10:28 PM Andy Shevchenko
> > <andy.shevchenko@...il.com> wrote:
> > > My point is that this is clear abuse of the spec and:
> > > 1) we have to enable the broken, because it is already in the wild with
> > >    the comment that this is an issue
> > >
> > > AND
> > >
> > > 2) issue an ECR / work with MS to make sure they understand the problem.
> > >
> > > This can be done in parallel. What I meant as a prerequisite is to start doing
> > > 2) while we have 1) on table.
> >
> > Oh, okay, that makes sense. If you want to get (2) going, by all means
> > go for it. I have no idea how to do this myself; Ard said something
> > about joining the UEFI forum as an individual something or another but
> > I don't think I'm the man for the job there. Is this something that
> > Intel can do with their existing membership (is that the right term?)
> > at the UEFI forum? Or maybe a Microsoft engineer on the list?
>
> My team at Microsoft, which works on Linux, filed a bug on this
> issue against the Hyper-V team about a year ago, probably when the issue
> was raised during the previous attempt to implement the functionality
> in Linux.  I've talked with the Hyper-V dev manager, and they acknowledge
> that the ACPI entry Hyper-V provides to guest VMs violates the spec.  But
> changing to an identifier that meets the spec is problematic because
> of backwards compatibility with Windows guests on Hyper-V that
> consume the current identifier.  There's no practical way to have Hyper-V
> provide a conformant identifier AND fix all the Windows guests out in
> the wild to consume the new identifier.   As a result, at this point Hyper-V
> is not planning to change anything.
>
> It's a lousy state-of-affairs, but as mentioned previously in this thread,
> it seems to be one that we will have to live with.
>

Thanks for chiming in.

Why not do something like

Name (_CID, Package (2) { "VM_GEN_COUNTER", "VMGENCTR" } )

?

That way, older clients can match on the existing _CID and new clients
can match on the spec compliant one.