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From: Gerd Hoffmann <kraxel@redhat.com>
Date: Mon, 28 Nov 2016 10:01:57 +0100
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Subject: Re: [Qemu-devel] [PATCH v3 for-2.9 0/3] q35: add negotiable
 broadcast SMI
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To: Laszlo Ersek <lersek@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>, Kevin O'Connor <kevin@koconnor.net>, qemu devel list <qemu-devel@nongnu.org>, Igor Mammedov <imammedo@redhat.com>, "Jordan
	Justen (Intel address)" <jordan.l.justen@intel.com>, Michael Kinney <michael.d.kinney@intel.com>

  Hi,

> If I understand correctly, one argument against the current state of
> writeable fw_cfg, captured in
> <https://www.mail-archive.com/qemu-devel@nongnu.org/msg354983.html>, is
> that callbacks on write are not supported. Apparently, QEMU code that
> uses the data written by the guest is supposed to just read that data,
> not to expect a notification about it.
>=20
> I'm unsure how this can work for actual negotiation, where the guest
> usually does a read/write/read cycle, and expects some kind of change
> between steps #2 and #3. I don't see how that can be implemented in QEMU
> without write callbacks (i.e. how QEMU can confirm or reject the
> negotiation attempt).

Do you actually need negotiation?  I think you only need to know
whenever broadcast-smi is supported, and the presence of the
etc/broadcast-smi (or however we name that) fw_cfg file indicates that.
If it is there just write true/false to it to enable/disable, and qemu
checks the field each time a smi is raised.

cheers,
  Gerd