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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Tue, 22 Mar 2022 23:06:30 +0100
Message-ID: <CAMj1kXEExWbD9imqNUr1RYRzJmbQX5i3CdG7MPseQh8Q=N1y9g@mail.gmail.com>
Subject: Re: [PATCH 2/3 v6] ACPI: allow longer device IDs
To:     "Michael Kelley (LINUX)" <mikelley@microsoft.com>
Cc:     jason <jason@zx2c4.com>,
        Andy Shevchenko <andy.shevchenko@gmail.com>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-crypto <linux-crypto@vger.kernel.org>,
        ACPI Devel Maling List <linux-acpi@vger.kernel.org>,
        Alexander Graf <graf@amazon.com>,
        Mika Westerberg <mika.westerberg@linux.intel.com>,
        Hans de Goede <hdegoede@redhat.com>,
        Len Brown <lenb@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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On Tue, 22 Mar 2022 at 20:59, Michael Kelley (LINUX)
<mikelley@microsoft.com> wrote:
>
> From: Ard Biesheuvel <ardb@kernel.org> Sent: Monday, February 28, 2022 2:47 PM
> >
> > On Mon, 28 Feb 2022 at 23:38, Michael Kelley (LINUX)
> > <mikelley@microsoft.com> wrote:
> > >
> > > From: Ard Biesheuvel <ardb@kernel.org> Sent: Monday, February 28, 2022 2:22 PM
> > > >
> > > > On Mon, 28 Feb 2022 at 23:14, Michael Kelley (LINUX)
> > > > <mikelley@microsoft.com> wrote:
> > > > >
> > > > > From: Jason A. Donenfeld <Jason@zx2c4.com> Sent: Monday, February 28, 2022
> > > > 1:55 PM
> > > > > >
> > > > > > Hi Andy,
> > > > > >
> > > > > > On Mon, Feb 28, 2022 at 10:28 PM Andy Shevchenko
> > > > > > <andy.shevchenko@gmail.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.
> > >
> > > I'll run this by the Hyper-V guys.  I don't have the ACPI expertise to disagree
> > > with them when they say they can't change it. :-(
> > >
> >
> > Yes, please, even if it makes no difference for this particular patch.
>
> The Hyper-V guys pass along their thanks for your suggestion.  They have
> created an internal build with the change and verified that it preserves
> compatibility with Windows guests.  I've tested with Linux guests and
> Jason's new driver (modified to look for "VMGENCTR"), and it all looks good.
> It will take a little while to wend its way through the Windows/Hyper-V
> release system, but they are planning to take the change.
>

Thanks for reporting back.

Will the spec be updated accordingly?