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From: Gerd Hoffmann <kraxel@redhat.com>
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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: 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>, Marc =?ISO-8859-1?Q?Mar=ED?= <markmb@redhat.com>, Laszlo <lersek@redhat.com>

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.

cheers,
  Gerd