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From: Laszlo Ersek <lersek@redhat.com>
Message-ID: <55E6BFA1.8020402@redhat.com>
Date: Wed, 2 Sep 2015 11:21:37 +0200
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Subject: Re: [Qemu-devel] [PATCH v2 1/5] fw_cfg: document
 fw_cfg_modify_iXX() update functions
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To: Gerd Hoffmann <kraxel@redhat.com>, Peter Maydell <peter.maydell@linaro.org>
Cc: Drew <drjones@redhat.com>, Stefan Hajnoczi <stefanha@gmail.com>, "Gabriel L. Somlo" <somlo@cmu.edu>, QEMU Developers <qemu-devel@nongnu.org>, Kevin O'Connor <kevin@koconnor.net>, =?UTF-8?Q?Marc_Mar=c3=ad?= <markmb@redhat.com>

On 09/02/15 10:08, Gerd Hoffmann wrote:
> On Di, 2015-09-01 at 21:10 +0100, Peter Maydell wrote:
>> On 1 September 2015 at 20:13, Gabriel L. Somlo <somlo@cmu.edu> wrote:
>>> Also, since I'll be tinkering with fw_cfg again, and you mentioned
>>> using DT on arm and ACPI on x86 to auto-detect the presence (and location)
>>> of fw_cfg from the guest-side in a related thread:
>>
>> I meant DT or ACPI on ARM, actually. I don't know what the x86
>> approach is for fw_cfg but I think it's just "known address".
> 
> Yes, "known address".  And given that the firmware actually loads the
> acpi tables via fw_cfg that is very unlikely to change.  Adding it to
> the acpi tables might be useful nevertheless so the guest os knows the
> ioports are in use.
> 
> I think on arm the acpi situation is the same, but I think firmware
> detecting the location via DT should work without chicken&egg problems.

First of all I apologize for lagging severely behind this thread; in
practice I can't read emails longer than 20 lines or so. This one
qualifies, so I'll respond:

Yes, on qemu-system-(arm|aarch64) -M virt, detecting the fw_cfg
registers via DT should continue to work without any dependencies.

Thanks
Laszlo