Re: [PATCH v7 16/32] xen/x86: allow disabling the pmtimer

["Roger Pau Monné" <roger.pau@citrix.com>]

El 03/11/15 a les 13.41, Jan Beulich ha escrit:
>>>> On 03.11.15 at 11:57, <andrew.cooper3@citrix.com> wrote:
>> On 03/11/15 07:21, Jan Beulich wrote:
>>>>>> On 30.10.15 at 16:36, <andrew.cooper3@citrix.com> wrote:
>>>> On 30/10/15 13:16, Jan Beulich wrote:
>>>>>>>> On 30.10.15 at 13:50, <roger.pau@citrix.com> wrote:
>>>>>> El 14/10/15 a les 16.37, Jan Beulich ha escrit:
>>>>>>>>>> On 02.10.15 at 17:48, <roger.pau@citrix.com> wrote:
>>>>>>>> Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
>>>>>>>> Cc: Jan Beulich <jbeulich@suse.com>
>>>>>>>> Cc: Andrew Cooper <andrew.cooper3@citrix.com>
>>>>>>>> ---
>>>>>>>> Changes since v6:
>>>>>>>>  - Return ENODEV in pmtimer_load if the timer is disabled.
>>>>>>>>  - hvm_acpi_power_button and hvm_acpi_sleep_button become noops if the
>>>>>>>>    pmtimer is disabled.
>>>>>>> But how are those two features connected? I don't think you can
>>>>>>> assume absence of a PM block just because there's no PM timer.
>>>>>>> Or if you want to tie them together for now, the predicate needs
>>>>>>> to be renamed.
>>>>>>>
>>>>>>>>  - Return ENODEV if pmtimer_change_ioport is called with the pmtimer
>>>>>>>>    disabled.
>>>>>>> Same here.
>>>>>> What about changing XEN_X86_EMU_PMTIMER into XEN_X86_EMU_PM and this
>>>>>> flags disables all PM stuff?
>>>>> Ah, right, that's a reasonable option.
>>>> It still might be a nice idea to split them in two, given future work.
>>>>
>>>> To support hotplug properly (cpu, ram and pci), Xen needs to inject
>>>> GPEs, which comes from part of the PM infrastructure.  To support PCI
>>>> devices in the future without the whole PM infrastructure, it would be
>>>> nice to keep the split.
>>> Coming back to this - I'm not sure: The hotplug aspect as you
>>> mention it should matter for Dom0 only. DomU could (and perhaps
>>> should) use a PV interface instead.
>>
>> I disagree.
>>
>> All PVH guests should use the same mechanism; making a split between
>> dom0 and domU will only make our lives harder.
>>
>> Where reasonable, we should follow what happens on native; one of the
>> underlying points of PVH is to have less of an impact on the guest
>> side.  In some cases it is indeed nasty, but has the advantage of being
>> well understood.
> 
> What meaning would ACPI have to a PVH DomU?
> 
>>> So I'd like to suggest quite the opposite: Don't call the thing PM,
>>> but make it more general and call it ACPI. And instead of
>>> separating HPET, we might have this fall under ACPI as well, or
>>> we might have a second TIMER flag, requiring both to be set
>>> for there to be a HPET and PMTMR. This leaves open the option
>>> of Dom0 getting ACPI enabled (despite this then being "real",
>>> not emulated ACPI), but TIMER left off.
>>
>> An HPET can exist independently of other features such as ACPI.  It
>> should have its own option.
> 
> Without ACPI there's no defined way to discover it. Doing what
> Linux does - applying chipset knowledge - won't work on PVH either,
> because there's no emulated chipset. Which would leave scanning
> physical memory, but if there is none, none can be found.
> 
>> +1 to having an ACPI option, but as indicated above, I expect it to be
>> used in the longterm even for domU.
> 
> Again - why and how?

I think that at this point in the design it's not so important to have
all the XEN_X86_EMU_* properly defined. This is not a public interface,
so we can expand/reduce them whenever we want. Would it be fine, for the
time being to just have a XEN_X86_EMU_PM and control both the PM and the
PMTMR?

Roger.


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