Re: [Xen-users] Status of PVSCSI

["Austin S. Hemmelgarn" <ahferroin7@xxxxxxxxx>]

On 2016-08-11 13:15, Jon Schewe wrote:
> On 8/11/16 12:07 PM, Austin S. Hemmelgarn wrote:
> > On 2016-08-11 11:47, Jon Schewe wrote:
> > > On 8/5/16 10:38 AM, Jon Schewe wrote:
> > > > On 8/5/16 10:27 AM, Simon Hobson wrote:
> > > > > Jon Schewe <jschewe@xxxxxxx> wrote:
> > > > >
> > > > > > I'm looking to move my backups into a VM and PVSCSI seems like the
> > > > > > right way to get the tape library and drive visible to the VM.
> > > > > > ...
> > > > > > So what front end can I use pvscsi with?
> > > > > Can you use PCI passthrough of the SCSI controller ?
> > > > I have considered that, however I have a couple of issues with it:
> > > > 1) When I tested it, after shutting down the domU the dom0 couldn't
> > > > attach to the SCSI controller again until I rebooted. Perhaps that was
> > > > because I shutdown the domU before calling pci-detach.
> > > >
> > > > 2) I see that I need to detach the pci devices before save and restore.
> > > > That will make things hard when rebooting the dom0 for updates. I
> > > > usually just run "service xendomains stop" and then run the updates and
> > > > reboot. This suspends the domUs and everything is fine on startup. If
> > > > this process will automatically handle the detach and attach calls,
> > > > then
> > > > I may be able to use it.
> > > Does anyone know if pci devices need to be explicitly detached before
> > > shutdown or if xen will do the right thing on shutdown of the VM?
> > In my experience, it depends on the device.  Some devices work fine
> > (mostly really simple dumb devices, such as some really cheap GbE
> > NIC's), but many need hardware re-initialization before they will work
> > (and some like certain GPU's actually need a full power cycle before
> > they work reliably).  For many of those that need hardware
> > re-initialization,, you can unload and reload the driver in Domain-0
> > to reinitialize the hardware, but only after you've detached the
> > device from the VM and removed the reservations.
> > > Also if I call "/etc/init.d/xendomains stop" and the VMs are saved, will
> > > that do the right thing or will I need to manually detach pci devices
> > > that are passed through?
> > You would have to detach the devices if you want to use them elsewhere
> > before restoring the VM.  Think along the same lines as accessing the
> > VM's disk image while it's stopped, the OS won't be expecting the
> > change, and it may lead to data loss or other odd behavior.  Saving a
> > VM is essentially the same in this case as putting a laptop to sleep,
> > so you really should avoid doing anything with any hardware for the VM
> > (real or emulated) when it's saved and not running, and more
> > importantly, don't power cycle any of the hardware attached to it
> > until it's been shut down properly.
> In my use case I'm thinking that I would dedicate the PCI device (SCSI
> controller here) to one VM and not  move it to another VM or domain-0.
> Given that assumption, it should all work pretty seamlessly, correct?
It generally should, but like I said, you might have issues if you leave 
the device attached while the machine is saved and then reboot the host 
(or the host crashes, or otherwise has to restart).  I have no personal 
experience passing through SCSI controllers here though, just NIC's 
(which usually have no issues), so I there may be involved factors that 
I don't know about.
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