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From: Gerd Hoffmann <kraxel@redhat.com>
Date: Wed, 01 Jul 2015 14:30:29 +0200
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Subject: Re: [Qemu-devel] [PATCH v2 02/22] virtio: run drivers in 32bit mode
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: seabios@seabios.org, qemu-devel@nongnu.org

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@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