On Mon, Jun 01, 2015 at 10:58:57AM +0200, Paolo Bonzini wrote: > > > 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.
Yes, I got that. But I guess typically only one CPU is running so something like smm_enabled_any will DTRT, without layering violations, right? > 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 To me it seems easier to support than to explain why we don't have to, but if you are strongly against that, please add code comments explaining all this. -- MST
