[SeaBIOS] [PATCH v2 02/22] virtio: run drivers in 32bit mode

[Michael S. Tsirkin]

On Wed, Jul 01, 2015 at 03:50:50PM +0200, Michael S. Tsirkin wrote:
> On Wed, Jul 01, 2015 at 02:30:29PM +0200, Gerd Hoffmann wrote:
> > On Mi, 2015-07-01 at 10:08 +0200, Michael S. Tsirkin wrote:
> > > On Tue, Jun 30, 2015 at 10:38:53AM +0200, Gerd Hoffmann wrote:
> > > > virtio version 1.0 registers can (and actually do in the qemu
> > > > implementation) live in mmio space.  So we must run the blk and
> > > > scsi virtio drivers in 32bit mode, otherwise we can't access them.
> > > > 
> > > > This also allows to drop a bunch of GET_LOWFLAT calls from the virtio
> > > > code in the following patches.
> > > > 
> > > > Signed-off-by: Gerd Hoffmann <kraxel at redhat.com>
> > > 
> > > Is there an advantage to running them in a 16 bit mode?
> > 
> > Not really any more.  Switching from 32bit mode back to
> > whatever-was-active-before used to be problematic before we had smm mode
> > support.  In theory.  Because you can't save/restore the complete x86
> > processor state.  In practice we had surprisingly few problems,
> > appearently linux boot loaders simply don't play dirty tricks.
> > 
> > cheers,
> >   Gerd
> > 
> 
> Interesting. Might not be true for non-linux loaders :)
> 
> Anyway we support SSM now so all should be well, right?

Also a question: what's cheaper on kvm: use SMM to save/restore
or access device through the config cap?

> -- 
> MST