Re: [Qemu-devel] [PATCH v2] hw/isa/lpc_ich9: inject SMI on all VCPUs if APM_STS == 'Q'

[Laszlo Ersek]

On 11/16/16 15:05, Paolo Bonzini wrote:
> 
> 
> On 16/11/2016 14:18, Michael S. Tsirkin wrote:
>>> - we could have another magic 0xB2 value, which is implemented directly
>>> in QEMU and sets 0xB3 to a magic value.  Then OVMF can invoke it
>>> after SMBASE relocation and SMM IPL (so as not to crash on old QEMUs)
>>> to detect the new feature.  It can fail to start if using traditional
>>> AP and the new feature is not there.
>>
>> If we keep collecting these magic values, should architect it
>> and do a host/guest bitmap like virtio does?
> 
> The value written in 0xB3 can certainly be a feature bitmap.  For now we
> would have for example
> 
> bit 0 if set, writing 0x10-0xFF to 0xB2 results in a broadcast SMI
> bit 1-7       zero

Doable, but:
- doesn't address how OVMF learns about the broadcast SMI availability,
- the command value OVMF currently writes is 0.

How about this:
- etc/smi/features is the LE uint64_t bitmap proposed earlier, bit#0
stands for broadcast SMI availability
- 0xB2 is the command value (independent of 0xB3)
- 0XB3 is a guest feature bitmap (valid for the next request). SeaBIOS
reserves bit#0 already (uses values 0 and 1), so we can use the
remaining 7 bits for requesting features. Bit#1 (value 2) could be the
broadcast SMI.

This does resemble a kind of feature negotiation, except the host cannot
signal back an error (unsupported combination of features), like
virtio-1.0 can. We can make QEMU abort in that case, or ignore the flags.

Thanks
Laszlo