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

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

On 05/26/2014 05:33 AM, Jan Beulich wrote:
>>>> On 23.05.14 at 17:20, <andrew.cooper3@citrix.com> 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.
> I don't see what 4.5 has to do with this - 4.4 already has PVH
> support (it being experimental and DomU only imo doesn't matter
> as far as feature reporting is concerned).
>
>> 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.
> Depends how you view it - if this becomes indistinguishable from
> PVH for the tools stack, it could also get dropped again. Otoh I
> don't think it will (or even should) become indistinguishable, and
> hence I'm not sure its functional folding with (most of) HVM would
> actually be a valid reason to drop this indication again (or, if it
> has to remain, to consider it deprecated and pointless).


Currently PVH requires, for example, VMX's secondary exec controls. I 
don't know whether the plan is to drop this requirement eventually but 
if not then some processors might not be able to run PVH.

-boris