Re: [PATCH] virtio-blk: drop deprecated scsi=on|off property

[Eduardo Habkost]

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?

This looks like a sign we should have deprecated pc-2.4 a long
time ago.

> 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.


> I have tested that libvirt still works when the property is absent. It
> no longer adds scsi=on|off to the command-line.
> 
> Cc: Markus Armbruster <armbru@redhat.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Peter Krempa <pkrempa@redhat.com>
> Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
[...]
> diff --git a/hw/core/machine.c b/hw/core/machine.c
> index 40def78183..286f18ec6d 100644
> --- a/hw/core/machine.c
> +++ b/hw/core/machine.c
> @@ -194,8 +194,6 @@ GlobalProperty hw_compat_2_5[] = {
>  const size_t hw_compat_2_5_len = G_N_ELEMENTS(hw_compat_2_5);
>  
>  GlobalProperty hw_compat_2_4[] = {
> -    /* Optional because the 'scsi' property is Linux-only */
> -    { "virtio-blk-device", "scsi", "true", .optional = true },
>      { "e1000", "extra_mac_registers", "off" },
>      { "virtio-pci", "x-disable-pcie", "on" },
>      { "virtio-pci", "migrate-extra", "off" },


-- 
Eduardo