Re: [Qemu-devel] [Qemu-block] Behavior of QMP "query-block"

[Kevin Wolf]

Am 25.05.2017 um 20:44 hat Bruno Alvisio geschrieben:
> Hello John,
> 
> Thanks. Yes, typo when I wrote the e-mail.
> 
> It might be possible that XEN does that (I will ask in the XEN forum). 
> However,
> I have noticed that it is not the case for all of the VMs I have launched. In
> some of them I can query the block devices even after a long time the VM has
> been running.
> 
> I was wondering if the device is removed in case the disk is corrupted or
> something. When I mention it runs normally, I can log into ithe VM and perform
> read/write operations such creating files.
> 
> Also, is there an easy way to log all qemu events so that I can see what might
> be going on with the 'ide' device?

There is one completely crazy thing that Xen does with respect to disks.
Instead of having support for their PV disks (i.e. virtio-blk, just
different) in the BIOS, they add _both_ an IDE disk and a PV disk to the
VM, so that the bootloader or non-PV-aware guest OSes can access the IDE
disk, for which they most certainly do have drivers. As soon as a driver
for the PV disk is loaded, however, that driver calls a hypervisor
function that removes all the IDE disks from the VM and leaves only the
PV ones there, so that the PV-aware guest doesn't see two same disks.

I suspect that what you're seeing initially is the IDE disks, and when
the PV driver is loaded, they disappear.

Kevin