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Re: [Xen-devel] [PATCH] x86: Expose hypervisor's PVH support via xen_caps

On 5/28/2014 5:42 AM, Ian Campbell wrote:
On Fri, 2014-05-23 at 11:53 -0400, Boris Ostrovsky wrote:
On 05/23/2014 11:35 AM, Roger Pau Monnà wrote:
On 23/05/14 17:32, Boris Ostrovsky wrote:
On 05/23/2014 11:20 AM, Andrew Cooper wrote:
On 23/05/14 16:08, Boris Ostrovsky wrote:
On 05/23/2014 11:00 AM, Andrew Cooper wrote:
On 23/05/14 15:55, Boris Ostrovsky wrote:
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
     xen/arch/x86/setup.c | 5 +++++
     1 file changed, 5 insertions(+)
If the plan is to try and PVH and HVM back into one mode as far as Xen
is concerned, doesn't this become redundant?
Yes, I was thinking about this but we currently don't have (or,
rather, I can't think of) a good way to determine whether we can start
a PVH guest. We can grep the log but that doesn't feel like a
particularly good solution.

One option could be to postpone this patch until 4.5 freezes and see
whether we indeed followed up on the plan and if we didn't then
integrate it.

My concern here is that if this patch gets accepted, it will have to say
forever more as the cap strings are a very public API.
Yes, that's true.

The problem that we have now is that if we have 'pvh=1' in the config
file the guest will fail to start if PVH is not on. Can we, for example,
revert (with a warning) to pure PV if that's the case?
Won't this option go away once PVH is stable, so the toolstack can
detect if the kernel supports PVH and start the guest in this mode by
default? (of course falling back to PV if PVH is not supported).
Yes, it will, by virtue of hypervisor never having to take this action
(reverting to PV) since PVH would be stable and always supported.
This ignores hardware which cannot support pvh and guest kernels which
are lacking the support for it.

I just don't know whether ignoring pvh=1 directive on systems where PVH
is not supported is acceptable. (Your "of course" seems to indicate that
you think it is.)
The ideal case would be that you don't say pvh=anything and the
toolstack will automatically use pvh if the h/w and kernel both support
it, otherwise it will use pv.

But don't we want to let the user now whether PVH is supported? Could it be that they may only want to start a guest in PVH mode? (I realize that I am contradicting my own suggestion to ignore pvh=1).


If you say pvh=1 then the toolstack should unconditionally attempt to
create a pvh guest, and if the hypervisor rejects an attempt to create
such a domain then it should fail, because this is what you have asked
for with pvh=1.

Then we need some sort of indication of PVH is support so that users know whether they can supply pvh=1.

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