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

[boris ostrovsky <boris.ostrovsky@oracle.com>]

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 Monné 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@oracle.com>
>>>>>>>> ---
>>>>>>>>      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.
>>>>>>
>>>>>> -boris
>>>>> 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).

-boris

>
> 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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