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

[John Snow <jsnow@redhat.com>]

On 05/26/2017 07:40 AM, Kevin Wolf wrote:
> 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
> 

Sure, but does that mean that you'd see *no* block devices via a query
afterwards?