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

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Nov 17, 2016 at 02:16:35PM +0100, Laszlo Ersek wrote:
> On 11/16/16 21:27, Michael S. Tsirkin wrote:
> > On Wed, Nov 16, 2016 at 07:03:27PM +0100, Laszlo Ersek wrote:
> >> 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
> > 
> > I think that if you are going to do it, do it like 1.0:
> > - same bitmap for host and guest. how about a writeable fw cfg file?
> 
> fw_cfg files are not writeable since qemu 2.4 (see commits 023e3148567ac
> and 6cec43e178cde).
> 
> How about this alternative, in STS:
> - bit 0: read and written transparently
> - bit 1: on write:
>          0 -- set features in bits 2-7
>          1 -- query host features into bits 2-7
>          on read:
>          - after querying features:
>            - reads back as 0 if the interface is supported
>            - reads back as 1 if the interface is missing
>          - after setting features:
>            - reads back as 0 if the feature subset is valid
>            - reads back as 1 otherwise
> - bit 2: on write:
>          - when setting features: request broadcast SMI
>          - when querying features: ignored
>          on read:
>          - after setting features: zero
>          - after querying features: broadcast SMI availability (1 if
>            available)
> 
> - bit 3-7: future features (I think 5 more features for SMI handling
>            should suffice), working similarly to bit 2
> 
> SeaBIOS writes values 0x00 and 0x01, and expects to find the same when
> reading back. Bit pattern 0000_000?b  translates to "clear all
> features", which always succeeds and results in behavior identical to
> the current one, hence bits 1-7 read back as zero.
> 
> OVMF:
> - write 0x02, read back value:
>   - if bit 1 is set, interface is missing
>   - otherwise feature bitmap was returned in bits 2-7
> - select requested features in bits 2-7, set bit 1 to 0, write value,
>   read back value
>   - if bit 1 is set, the feature subset is invalid
>   - okay otherwise
> 
> Thanks
> Laszlo


It's all fine, or we can make fw cfg writeable again (I posted
a patch for that a while ago), but it's all a bit too much
for this release.

Let's just defer it, or do you have a better idea?

> > - use 0XB3 bit for FEATURES_OK
> >