Re: [Xen-users] PV vs HVM for PCI-Passthrough

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

If you aren't planning to use any passthrough on the older staging
machine then PV or HVM isn't a concern, you can choose based on whatever
other needs you have, the wiki has some guides on the features and
strengths/weaknesses of the various modes, see e.g. 
http://wiki.xen.org/wiki/Xen_Project_Software_Overview
http://wiki.xen.org/wiki/Understanding_the_Virtualization_Spectrum
http://wiki.xen.org/wiki/Virtualization_Spectrum

As for migration it depends on what you mean. If you just mean "shut
down the VM, copy all its various bits to another machine and start it",
then you should be absolutely fine.

If however you mean migrate in the sense of a live migration, or a
suspend on one machine resume on the other, then you will need to worry
about CPU feature compatibility between the two machines, and perhaps do
cpuid levelling on the more capable one, either via the xl.cfg cpuid=
option on a per-guest basis or the cpu_mask_* hypervisor command line
options to level the whole host.

Lastly wrt:
> * PV only supports passthrough on non iommu capable machines

PV supports passthrough regardless of the presence of an IOMMU, it just
doesn't *require* one. Things will be better (more secure) if you do
have an IOMMU but will work without.

HVM does require an IOMMU for passthrough.

Ian.

On Tue, 2014-12-16 at 10:38 -0500, symack wrote:
> Ian thank you for your response. Maybe if I take this a step back I
> can better explain what
> I am trying to achieve.
> 
> 
> I need to create DomU  instances on a test server (non iommu capable),
> and eventually
> transfer the virutal machines to a shiny new server (iommu capable),
> and setup passthough.
> 
> 
> Should I be creating HVM or PV instances on the test server? Which is
> the best approach
> for easy migration.
> 
> 
> If I can jump the gun, judging by what you are saying:
> 
> 
> * PV only supports passthrough on non iommu capable machines
> 
> * HVM support passthrough on iommu capable machines
> 
> 
> My question is then, is it best for me to just create HVM domus on the
> test server, with the intention
> to eventually migrate the virutual machines to an iommu environment.
> Is this possible?
> 
> 
> Thanks in Advance!
> 
> 
> Nick from Toronto.
> 
> 
> 
> 
> 
> 
> 
> Currently the machine has no iommu capability however, instead of
> creating PV guests on a
> non iommu capable box, we are planning to create hvm domu instances,
> with the hopes that we
> can transfer the virtual machines to the new box (iommu capable) when
> the time comes. 
> 
> 
> A little modification, the installation of some native drivers.... 
> â



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