Re: [Qemu-devel] [PATCH v3 for-2.9 0/3] q35: add negotiable broadcast SMI

[Laszlo Ersek]

On 11/28/16 10:01, Gerd Hoffmann wrote:
>   Hi,
> 
>> If I understand correctly, one argument against the current state of
>> writeable fw_cfg, captured in
>> <https://www.mail-archive.com/address@hidden/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.
>>
>> 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.

Michael suggested to use negotiation like virtio does (where the host
can reject invalid combinations of requested features):

http://lists.nongnu.org/archive/html/qemu-devel/2016-11/msg03077.html
http://lists.nongnu.org/archive/html/qemu-devel/2016-11/msg03218.html

I thought negotiation was overkill:

http://lists.nongnu.org/archive/html/qemu-devel/2016-11/msg03123.html

but I figured I'd just stick with the recommendation.

> If it is there just write true/false to it to enable/disable, and qemu
> checks the field each time a smi is raised.

If we agree there's no need for negotiation, I'm game.

Thanks
Laszlo