On Thu, Apr 29, 2021 at 02:03:52PM -0400, Eduardo Habkost wrote: > On Thu, Apr 29, 2021 at 04:52:21PM +0100, Stefan Hajnoczi wrote: > > The scsi=on|off property was deprecated in QEMU 5.0 and can be removed > > completely at this point. > > > > Drop the scsi=on|off option. It was only available on Legacy virtio-blk > > devices. Linux v5.6 already dropped support for it. > > > > Remove the hw_compat_2_4[] property assignment since scsi=on|off no > > longer exists. Old guests with Legacy virtio-blk devices no longer see > > the SCSI host features bit. > > > > This means pc-2.4 will now break guest ABI if using virtio-blk > devices, correct? Yes. However, this feature was only enabled on Linux hosts, so cross-host OS migration was always broken and no one noticed. Maybe that configuration is too niche and QEMU never supported cross-host OS migration, but it still means that the "pc-2.4" ABI was never solid to begin with :). > > > Live migrating old guests from an old QEMU with the SCSI feature bit > > enabled will fail with "Features 0x... unsupported. Allowed features: > > 0x...". We've followed the QEMU deprecation policy so users have been > > warned... > > > > Were they really warned, though? People running > "-machine pc-i440fx-2.4" might be completely unaware that it was > silently enabling a deprecated feature. > > Can we have this documented in a more explicit way? Maybe just a > comment at hw_compat_2_4 would be enough, to warn people doing > backports and rebases downstream. > > Can we make QEMU refuse to start if using pc-2.4 + virtio-blk > together, just to be sure? > > An alternative would be keeping the property (and the > hw_compat_2_4 entry) just to keep pc-2.4 working (until pc-2.4 is > deprecated and removed), but refusing to realize the device if > the feature is enabled. Yes, the least invasive approach is to leave the property in place but refuse to realize the virtio-blk device when scsi=on. The cost is more cruft, including a useless scsi=off command-line option that will continue to show up in libvirt-generated QEMU command-lines. The cautious approach makes sense to me and I'll send a new revision. Stefan
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