Re: [Xen-users] Does xen-fbfront on a DomU make use of a hardware GPU on Dom0?

[Simon Hobson <linux@xxxxxxxxxxxxxxxx>]

Roger Pau Monné <roger.pau@xxxxxxxxxx> wrote:

>>> The easiest way probably is to turn your PV guest into a HVM guest (which 
>>> will also improve performance) and then try to do PCI-passthrough of a 
>>> graphics card, although I'm not expert on GFX passthrough, so others might 
>>> have better advice.
>> HVM is faster than PV now? Can you point me at some numbers and
>> documentation on this? I've been considering changing from PV VMs to
>> HVM, but need good evidence to make it worth the time.
> 
> Here are a couple of pages that contain more information about PV vs HVM 
> (last one is a set of slides with performance numbers, although a little bit 
> outdated) :
> 
> https://wiki.xenproject.org/wiki/Understanding_the_Virtualization_Spectrum
> http://www.brendangregg.com/blog/2014-05-07/what-color-is-your-xen.html
> http://www.slideshare.net/xen_com_mgr/linux-pv-on-hvm

With the caveat that I'm probably even more confused about "modes" than I was, 
but those references don't (to me) say HVM is faster than PV. What they do 
suggest is that PV with HVM (what's being labelled as PVM) assistance is faster.

What I take away from those pages is that "pure PV" is still better than "pure 
HVM" - but hybrid modes are better still, using hardware support to do heavy 
lifting of the "difficult" bits of PV. If the hardware supports PVH then this 
is preferred, otherwise use HVM and there are now some optimisations that will 
bypass many of the performance hits that "pure" HVM has.
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