linux-kernel - Re: [PATCH v2] xen: Add support for XenServer 6.1 platform device

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Message-ID: <a14c6897-075c-4b2c-8906-75eb96d5c430@citrix.com>
Date: Thu, 27 Feb 2025 15:41:54 +0000
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: Frediano Ziglio <frediano.ziglio@...ud.com>,
 xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
 linux-pci@...r.kernel.org
Cc: Juergen Gross <jgross@...e.com>,
 Stefano Stabellini <sstabellini@...nel.org>,
 Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
 Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH v2] xen: Add support for XenServer 6.1 platform device

On 27/02/2025 2:50 pm, Frediano Ziglio wrote:
> On XenServer on Windows machine a platform device with ID 2 instead of
> 1 is used.
>
> This device is mainly identical to device 1 but due to some Windows
> update behaviour it was decided to use a device with a different ID.
>
> This causes compatibility issues with Linux which expects, if Xen
> is detected, to find a Xen platform device (5853:0001) otherwise code
> will crash due to some missing initialization (specifically grant
> tables). Specifically from dmesg
>
>     RIP: 0010:gnttab_expand+0x29/0x210
>     Code: 90 0f 1f 44 00 00 55 31 d2 48 89 e5 41 57 41 56 41 55 41 89 fd
>           41 54 53 48 83 ec 10 48 8b 05 7e 9a 49 02 44 8b 35 a7 9a 49 02
>           <8b> 48 04 8d 44 39 ff f7 f1 45 8d 24 06 89 c3 e8 43 fe ff ff
>           44 39
>     RSP: 0000:ffffba34c01fbc88 EFLAGS: 00010086
>     ...
>
> The device 2 is presented by Xapi adding device specification to
> Qemu command line.
>
> Signed-off-by: Frediano Ziglio <frediano.ziglio@...ud.com>

I'm split about this.  It's just papering over the bugs that exist
elsewhere in the Xen initialisation code.

But, if we're going to take this approach, then 0xc000 needs adding too,
which is the other device ID you might find when trying to boot Linux in
a VM configured using a Windows template.

Bjorn: to answer a prior question of yours, all 3 of these devices are
identical, and exist in production for political reasons (bindings in
Windows Updates) rather than technical reasons.

~Andrew