Re: [Qemu-devel] [PATCH 15/31] target-i386: use memory API to implement SMRAM

[Paolo Bonzini]

On 01/06/2015 10:10, Michael S. Tsirkin wrote:
> > UNUSED is written as zeroes, so it will think SMM is _disabled_,
> > unconditionally.  Note that d->smram_region is backwards: it aliases to
> > VRAM, so it is enabled when SMRAM is closed and disabled when SMRAM is open.
> > 
> > This is correct for KVM, though not for TCG.  Backwards migration is not
> > supported officially upstream, and I think we can agree it is even less
> > supported for TCG.
>
> Generally backwards migration is nice to have to test cross-version
> migration properly by doing ping-pong.
> Looks like we only need to set smm_enabled correctly before save,
> and it'll work cleanly.
> No?

Sort of.  Old QEMU is not able to handle the case where some CPUs are in
SMM and some are not.  It is also a layering violation to compute
smm_enabled from the CPUs.

Old SeaBIOS never uses SMM after POST, and executes a dozen instructions
or so inside SMRAM.  New SeaBIOS does use SMM after POST (e.g. during
grub), but it _literally_ executes two instructions inside SMRAM and
then jumps to the F-segment, so even if you do not migrate smm_enabled
the chance that something break is basically zero.

This is why I decided that it wasn't worth the complication (and the
layering violation).

Paolo