Re: [PATCH v4 2/4] x86/compat: Test both PV and PVH guests for compat mode

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

On 09/02/2015 04:08 AM, Jan Beulich wrote:
>>>> Boris Ostrovsky <boris.ostrovsky@oracle.com> 09/02/15 2:55 AM >>>
>> On 08/27/2015 12:01 PM, Jan Beulich wrote:
>>>>>> On 13.08.15 at 20:12, <boris.ostrovsky@oracle.com> wrote:
>>>> @@ -777,7 +777,7 @@ int arch_set_info_guest(
>>>>    
>>>>        /* The context is a compat-mode one if the target domain is compat-mode;
>>>>         * we expect the tools to DTRT even in compat-mode callers. */
>>>> -    compat = is_pv_32bit_domain(d);
>>>> +    compat = is_pv_32bit_domain(d) || is_pvh_32bit_domain(d);
>>> I continue to think that this should include a v->domain ==
>>> current->domain check (to match behavior for HVM guests
>>> from the tool stack perspective). Having looked at patch 4, I also
>>> can't see how the tool stack is being made expect a non-native
>>> guest context record in the 32-bit PVH case (i.e. I'd appreciate
>>> if you could point out where that hides).
>> For vcpu 0 current->domain is dom0 so I am not sure how this check would
>> work.
>>
>> For a 32-bit PVH guest the toolstack will place data into
>> vcpu_guest_context_x86_32_t (in vcpu_x86_32()) and so the hypervisor,
>> knowing that the guest is a compat one (based on the test above), will
>> access appropriate fields.
>>
>> This is not how HVM guests are started --- "classic" PVH behaves very
>> much like a PV guest, unlike what we are doing with no-dm PVH.
> And I believe this to be wrong, and potentially getting in the way of the no-dm
> work - Roger?
>
> As to the reference to vcpu_x86_32() - by its name alone it is already clear
> that this is after the determination of what bit width a guest to deal with, and
> looking at xc_dom_32_pae I still can't see why PVH guests would (intentionally
> and legitimately) be treated like PV rather than HVM ones (not to speak of the
> fact that I don't think 32-bit PVH is in any way limited to PAE).

The purpose of this series is to get 32-bit guests to parity with 
classic PVH with minimal changes and then move on to no-dm. Not getting 
in the way of no-dm is obviously important but making classic behave 
like no-dm (which is, to certain extent, is what you are suggesting) is 
out of scope.

(I haven't considered non-PAE case, TBH)

-boris