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 <kra...@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 :)
Without SMM, the only issue I've seen with "thunking" to 32bit mode was DOS-era programs (and in particular those that used emm386). > Anyway we support SSM now so all should be well, right? With SMM, I haven't seen any problems. I don't doubt that some DOS-era programs might still have issues though. Also, I haven't tested Paolo's kvm smm support yet. SeaBIOS already runs a number of drivers exclusively in 32bit mode: ahci, xhci, sdcard, ohci disks, pvscsi. Even without smm support, virtio is likely a good candidate to move to 32bit mode as I don't think there is a use case for running DOS-era programs on virtio disks. (Using 32bit only drivers results in smaller and easier to maintain code - indeed we'd like to move exclusively to 32bit drivers in the future.) Cheers, -Kevin
