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Date: Sat, 14 Dec 2013 09:25:12 +0100
From: =?ISO-8859-1?Q?Roger_Pau_Monn=E9?= <roger.pau@citrix.com>
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To: Colin Percival <cperciva@freebsd.org>, Mason Loring Bliss
 <mason@blisses.org>, <freebsd-xen@freebsd.org>
Subject: Re: XEN vs XENHVM?
References: <20131214022355.GX19296@blisses.org> <52ABC23E.4020408@freebsd.org>
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On 14/12/13 03:28, Colin Percival wrote:
> On 12/13/13 18:23, Mason Loring Bliss wrote:
>> I was psyched to see that GENERIC kernels in 10 have HVMXEN support by
>> default, but then I was left a little confused.
>>
>> What's the different between a kernel with options XEN and one with options
>> HVMXEN?
> 
> The XEN option is for *paravirtualized* Xen -- aka. the original version,
> before Intel and AMD added virtualization support into their CPUs.  HVM
> uses "hardware virtualization", but we also use PV drivers where available.
> 
>> I'd love to be able to run FreeBSD domU systems without having to do
>> a custom compile whenever there's an update. I've got a 9.1 system running
>> now, using a copy of the XEN config with a couple tweaks, and I see all the
>> PV drivers I expect. I'm wondering what's different with XENHVM... Also
>> useful would be knowing if there are remaining differences between i386 and
>> amd64 as a domU in FreeBSD 10.
> 
> You want to switch to using HVM with PV devices.  That should be a simple
> tweak to your Xen configuration, and then you'll be able to use a GENERIC
> kernel.

Just as a note, the support in GENERIC is not only HVM with PV devices,
is basically a PV guest inside an HVM container, meaning it also uses PV
IPIs and PV timers.

The main difference between pure PV and PVHVM is that PV requires a PV
MMU implementation in the OS, while PVHVM can use a hardware virtualized
MMU (because it's running inside of a HVM container).

Roger.