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Date:   Wed, 22 Jun 2022 16:50:21 -0500
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Subject: Re: [PATCH 1/2] x86: notify hypervisor about guest entering s2idle
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To:     Grzegorz Jaszczyk <jaz@semihalf.com>,
        Sean Christopherson <seanjc@google.com>
Cc:     linux-kernel@vger.kernel.org, Dmytro Maluka <dmy@semihalf.com>,
        Zide Chen <zide.chen@intel.corp-partner.google.com>,
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        Tomasz Nowicki <tn@semihalf.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
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        Hans de Goede <hdegoede@redhat.com>,
        Sachi King <nakato@nakato.io>,
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        Wei Wang <wei.w.wang@intel.com>,
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From:   "Limonciello, Mario" <mario.limonciello@amd.com>
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On 6/22/2022 04:53, Grzegorz Jaszczyk wrote:
> pon., 20 cze 2022 o 18:32 Limonciello, Mario
> <mario.limonciello@amd.com> napisał(a):
>>
>> 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).
> 
> It will be problematic since the abort/restore notification could
> arrive too late and therefore the whole system will go to suspend
> thinking that the guest is in desired s2ilde state. Also in this case
> it would be impossible to prevent races and actually making sure that
> the guest is suspended or not. We already had similar discussion with
> Sean earlier in this thread why the notification have to be send just
> before swait_event_exclusive(s2idle_wait_head, s2idle_state ==
> S2IDLE_STATE_WAKE) and that the VMM have to have control over guest
> resumption.
> 
> Nevertheless if extending acpi_s2idle_dev_ops is possible, why not
> extend it about the hypervisor_notify() and use it in the same place
> where the hypercall is used in this patch? Do you see any issue with
> that?

If this needs to be a hypercall and the hypercall needs to go at that 
specific time, I wouldn't bother with extending acpi_s2idle_dev_ops. 
The whole idea there was that this would be less custom and could follow 
a spec.

TBH - given the strong dependency on being the very last command and 
this being all Linux specific (you won't need to do something similar 
with Windows) - I think the way you already did it makes the most sense.
It seems to me the ACPI device model doesn't really work well for this 
scenario.

> 
>>
>>> 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.
> 
> Thank you for your explanation. This means that I should register
> "HYPE" through https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fuefi.org%2FPNP_ACPI_Registry&amp;data=05%7C01%7Cmario.limonciello%40amd.com%7C49512293908e4ee17e8c08da54351ed5%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637914884458918039%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&amp;sdata=v5VsnxAINiJhOMLpwORLHd13WcYBHf%2FGSNv8Bjhyino%3D&amp;reserved=0 before introducing
> this new driver to Linux.
> I have no experience with the above, so I wonder who should be
> responsible for maintaining such ACPI ID since it will not belong to
> any specific vendor? There is an example of e.g. COREBOOT PROJECT
> using "BOOT" ACPI ID [1], which seems similar in terms of not
> specifying any vendor but rather the project as a responsible entity.
> Maybe you have some recommendations?

Maybe LF could own a namespace and ID?  But I would suggest you make a 
mockup that everything works this way before you go explore too much.

Also make sure Rafael is aligned with your mockup.

> 
> I am also not sure if and where a specification describing such a
> device has to be maintained. Since "HYPE0001" will have its own _DSM
> so will it be required to document it somewhere rather than just using
> it in the driver and preparing proper ACPI tables for guest?
> 
>>
>>> 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.
>>
> 
> I do not need to worry about Microsoft guests.

Makes life a lot easier :)

> 
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> 
> Thank you,
> Grzegorz