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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 02.09.15 at 14:31, <boris.ostrovsky@xxxxxxxxxx> wrote:
> On 09/02/2015 04:08 AM, Jan Beulich wrote:
>>>>> Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx> 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@xxxxxxxxxx> 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.

Well, okay. But are you saying then that 64-bit PVH also just
_happens_ to be treated like PV in the tool stack? IOW I'm still missing
the explicit tool stack adjustment that makes it use a guest-bit-width
context for PVH guests.

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

But I'm afraid you need to.

Jan


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