Re: [PATCH 1/2] x86: notify hypervisor about guest entering s2idle state

["Limonciello, Mario" <mario.limonciello@amd.com>]

On 6/20/2022 10:43, Grzegorz Jaszczyk wrote:
> czw., 16 cze 2022 o 18:58 Limonciello, Mario
> <mario.limonciello@amd.com> napisał(a):
>>
>> On 6/16/2022 11:48, Sean Christopherson wrote:
>>> On Wed, Jun 15, 2022, Grzegorz Jaszczyk wrote:
>>>> pt., 10 cze 2022 o 16:30 Sean Christopherson <seanjc@google.com> napisał(a):
>>>>> MMIO or PIO for the actual exit, there's nothing special about hypercalls.  As for
>>>>> enumerating to the guest that it should do something, why not add a new ACPI_LPS0_*
>>>>> function?  E.g. something like
>>>>>
>>>>> static void s2idle_hypervisor_notify(void)
>>>>> {
>>>>>           if (lps0_dsm_func_mask > 0)
>>>>>                   acpi_sleep_run_lps0_dsm(ACPI_LPS0_EXIT_HYPERVISOR_NOTIFY
>>>>>                                           lps0_dsm_func_mask, lps0_dsm_guid);
>>>>> }
>>>>
>>>> Great, thank you for your suggestion! I will try this approach and
>>>> come back. Since this will be the main change in the next version,
>>>> will it be ok for you to add Suggested-by: Sean Christopherson
>>>> <seanjc@google.com> tag?
>>>
>>> If you want, but there's certainly no need to do so.  But I assume you or someone
>>> at Intel will need to get formal approval for adding another ACPI LPS0 function?
>>> I.e. isn't there work to be done outside of the kernel before any patches can be
>>> merged?
>>
>> There are 3 different LPS0 GUIDs in use.  An Intel one, an AMD (legacy)
>> one, and a Microsoft one.  They all have their own specs, and so if this
>> was to be added I think all 3 need to be updated.
> 
> Yes this will not be easy to achieve I think.
> 
>>
>> As this is Linux specific hypervisor behavior, I don't know you would be
>> able to convince Microsoft to update theirs' either.
>>
>> How about using s2idle_devops?  There is a prepare() call and a
>> restore() call that is set for each handler.  The only consumer of this
>> ATM I'm aware of is the amd-pmc driver, but it's done like a
>> notification chain so that a bunch of drivers can hook in if they need to.
>>
>> Then you can have this notification path and the associated ACPI device
>> it calls out to be it's own driver.
> 
> Thank you for your suggestion, just to be sure that I've understand
> your idea correctly:
> 1) it will require to extend acpi_s2idle_dev_ops about something like
> hypervisor_notify() call, since existing prepare() is called from end
> of acpi_s2idle_prepare_late so it is too early as it was described in
> one of previous message (between acpi_s2idle_prepare_late and place
> where we use hypercall there are several places where the suspend
> could be canceled, otherwise we could probably try to trap on other
> acpi_sleep_run_lps0_dsm occurrence from acpi_s2idle_prepare_late).
> 

The idea for prepare() was it would be the absolute last thing before 
the s2idle loop was run.  You're sure that's too early?  It's basically 
the same thing as having a last stage new _DSM call.

What about adding a new abort() extension to acpi_s2idle_dev_ops?  Then 
you could catch the cancelled suspend case still and take corrective 
action (if that action is different than what restore() would do).

> 2) using newly introduced acpi_s2idle_dev_ops hypervisor_notify() call
> will allow to register handler from Intel x86/intel/pmc/core.c driver
> and/or AMD x86/amd-pmc.c driver. Therefore we will need to get only
> Intel and/or AMD approval about extending the ACPI LPS0 _DSM method,
> correct?
>

Right now the only thing that hooks prepare()/restore() is the amd-pmc 
driver (unless Intel's PMC had a change I didn't catch yet).

I don't think you should be changing any existing drivers but rather 
introduce another platform driver for this specific case.

So it would be something like this:

acpi_s2idle_prepare_late
-> prepare()
--> AMD: amd_pmc handler for prepare()
--> Intel: intel_pmc handler for prepare() (conceptual)
--> HYPE0001 device: new driver's prepare() routine

So the platform driver would match the HYPE0001 device to load, and it 
wouldn't do anything other than provide a prepare()/restore() handler 
for your case.

You don't need to change any existing specs.  If anything a new spec to 
go with this new ACPI device would be made.  Someone would need to 
reserve the ID and such for it, but I think you can mock it up in advance.

> I wonder if this will be affordable so just re-thinking loudly if
> there is no other mechanism that could be suggested and used upstream
> so we could notify hypervisor/vmm about guest entering s2idle state?
> Especially that such _DSM function will be introduced only to trap on
> some fake MMIO/PIO access and will be useful only for guest ACPI
> tables?
> 

Do you need to worry about Microsoft guests using Modern Standby too or 
is that out of the scope of your problem set?  I think you'll be a lot 
more limited in how this can behave and where you can modify things if so.