Re: [PATCH v3 8/8] vfio/pci: Add the support for PCI D3cold state

Abhishek Sahu
2022-06-03T10:19:49Z

On 6/2/2022 11:14 PM, Alex Williamson wrote:
> On Thu, 2 Jun 2022 17:22:03 +0530
> Abhishek Sahu <abhsahu@nvidia.com> wrote:
>
>> On 6/1/2022 9:51 PM, Alex Williamson wrote:
>>> On Wed, 1 Jun 2022 15:19:07 +0530
>>> Abhishek Sahu <abhsahu@nvidia.com> wrote:
>>>
>>>> On 6/1/2022 4:22 AM, Alex Williamson wrote:
>>>>> On Tue, 31 May 2022 16:43:04 -0300
>>>>> Jason Gunthorpe <jgg@nvidia.com> wrote:
>>>>>
>>>>>> On Tue, May 31, 2022 at 05:44:11PM +0530, Abhishek Sahu wrote:
>>>>>>> On 5/30/2022 5:55 PM, Jason Gunthorpe wrote:
>>>>>>>> On Mon, May 30, 2022 at 04:45:59PM +0530, Abhishek Sahu wrote:
>>>>>>>>
>>>>>>>>> 1. In real use case, config or any other ioctl should not come along
>>>>>>>>> with VFIO_DEVICE_FEATURE_POWER_MANAGEMENT ioctl request.
>>>>>>>>>
>>>>>>>>> 2. Maintain some 'access_count' which will be incremented when we
>>>>>>>>> do any config space access or ioctl.
>>>>>>>>
>>>>>>>> Please don't open code locks - if you need a lock then write a proper
>>>>>>>> lock. You can use the 'try' variants to bail out in cases where that
>>>>>>>> is appropriate.
>>>>>>>>
>>>>>>>> Jason
>>>>>>>
>>>>>>> Thanks Jason for providing your inputs.
>>>>>>>
>>>>>>> In that case, should I introduce new rw_semaphore (For example
>>>>>>> power_lock) and move ‘platform_pm_engaged’ under ‘power_lock’ ?
>>>>>>
>>>>>> Possibly, this is better than an atomic at least
>>>>>>
>>>>>>> 1. At the beginning of config space access or ioctl, we can take the
>>>>>>> lock
>>>>>>>
>>>>>>> down_read(&vdev->power_lock);
>>>>>>
>>>>>> You can also do down_read_trylock() here and bail out as you were
>>>>>> suggesting with the atomic.
>>>>>>
>>>>>> trylock doesn't have lock odering rules because it can't sleep so it
>>>>>> gives a bit more flexability when designing the lock ordering.
>>>>>>
>>>>>> Though userspace has to be able to tolerate the failure, or never make
>>>>>> the request.
>>>>>>
>>>>
>>>> Thanks Alex and Jason for providing your inputs.
>>>>
>>>> Using down_read_trylock() along with Alex suggestion seems fine.
>>>> In real use case, config space access should not happen when the
>>>> device is in low power state so returning error should not
>>>> cause any issue in this case.
>>>>
>>>>>>> down_write(&vdev->power_lock);
>>>>>>> ...
>>>>>>> switch (vfio_pm.low_power_state) {
>>>>>>> case VFIO_DEVICE_LOW_POWER_STATE_ENTER:
>>>>>>> ...
>>>>>>> vfio_pci_zap_and_down_write_memory_lock(vdev);
>>>>>>> vdev->power_state_d3 = true;
>>>>>>> up_write(&vdev->memory_lock);
>>>>>>>
>>>>>>> ...
>>>>>>> up_write(&vdev->power_lock);
>>>>>>
>>>>>> And something checks the power lock before allowing the memor to be
>>>>>> re-enabled?
>>>>>>
>>>>>>> 4. For ioctl access, as mentioned previously I need to add two
>>>>>>> callbacks functions (one for start and one for end) in the struct
>>>>>>> vfio_device_ops and call the same at start and end of ioctl from
>>>>>>> vfio_device_fops_unl_ioctl().
>>>>>>
>>>>>> Not sure I followed this..
>>>>>
>>>>> I'm kinda lost here too.
>>>>
>>>>
>>>> I have summarized the things below
>>>>
>>>> 1. In the current patch (v3 8/8), if config space access or ioctl was
>>>> being made by the user when the device is already in low power state,
>>>> then it was waking the device. This wake up was happening with
>>>> pm_runtime_resume_and_get() API in vfio_pci_config_rw() and
>>>> vfio_device_fops_unl_ioctl() (with patch v3 7/8 in this patch series).
>>>>
>>>> 2. Now, it has been decided to return error instead of waking the
>>>> device if the device is already in low power state.
>>>>
>>>> 3. Initially I thought to add following code in config space path
>>>> (and similar in ioctl)
>>>>
>>>> vfio_pci_config_rw() {
>>>> ...
>>>> down_read(&vdev->memory_lock);
>>>> if (vdev->platform_pm_engaged)
>>>> {
>>>> up_read(&vdev->memory_lock);
>>>> return -EIO;
>>>> }
>>>> ...
>>>> }
>>>>
>>>> And then there was a possibility that the physical config happens
>>>> when the device in D3cold in case of race condition.
>>>>
>>>> 4. So, I wanted to add some mechanism so that the low power entry
>>>> ioctl will be serialized with other ioctl or config space. With this
>>>> if low power entry gets scheduled first then config/other ioctls will
>>>> get failure, otherwise low power entry will wait.
>>>>
>>>> 5. For serializing this access, I need to ensure that lock is held
>>>> throughout the operation. For config space I can add the code in
>>>> vfio_pci_config_rw(). But for ioctls, I was not sure what is the best
>>>> way since few ioctls (VFIO_DEVICE_FEATURE_MIGRATION,
>>>> VFIO_DEVICE_FEATURE_MIG_DEVICE_STATE etc.) are being handled in the
>>>> vfio core layer itself.
>>>>
>>>> The memory_lock and the variables to track low power in specific to
>>>> vfio-pci so I need some mechanism by which I add low power check for
>>>> each ioctl. For serialization, I need to call function implemented in
>>>> vfio-pci before vfio core layer makes the actual ioctl to grab the
>>>> locks. Similarly, I need to release the lock once vfio core layer
>>>> finished the actual ioctl. I have mentioned about this problem in the
>>>> above point (point 4 in my earlier mail).
>>>>
>>>>> A couple replies back there was some concern
>>>>> about race scenarios with multiple user threads accessing the device.
>>>>> The ones concerning non-deterministic behavior if a user is
>>>>> concurrently changing power state and performing other accesses are a
>>>>> non-issue, imo.
>>>>
>>>> What does non-deterministic behavior here mean.
>>>> Is it for user side that user will see different result
>>>> (failure or success) during race condition or in the kernel side
>>>> (as explained in point 3 above where physical config access
>>>> happens when the device in D3cold) ? My concern here is for later
>>>> part where this config space access in D3cold can cause fatal error
>>>> on the system side as we have seen for memory disablement.
>>>
>>> Yes, our only concern should be to prevent such an access. The user
>>> seeing non-deterministic behavior, such as during concurrent power
>>> control and config space access, all combinations of success/failure
>>> are possible, is par for the course when we decide to block accesses
>>> across the life of the low power state.
>>>
>>>>> I think our goal is only to expand the current
>>>>> memory_lock to block accesses, including config space, while the device
>>>>> is in low power, or some approximation bounded by the entry/exit ioctl.
>>>>>
>>>>> I think the remaining issues is how to do that relative to the fact
>>>>> that config space access can change the memory enable state and would
>>>>> therefore need to upgrade the memory_lock read-lock to a write-lock.
>>>>> For that I think we can simply drop the read-lock, acquire the
>>>>> write-lock, and re-test the low power state. If it has changed, that
>>>>> suggests the user has again raced changing power state with another
>>>>> access and we can simply drop the lock and return -EIO.
>>>>>
>>>>
>>>> Yes. This looks better option. So, just to confirm, I can take the
>>>> memory_lock read-lock at the starting of vfio_pci_config_rw() and
>>>> release it just before returning from vfio_pci_config_rw() and
>>>> for memory related config access, we will release this lock and
>>>> re-aquiring again write version of this. Once memory write happens,
>>>> then we can downgrade this write lock to read lock ?
>>>
>>> We only need to lock for the device access, so if you've finished that
>>> access after acquiring the write-lock, there'd be no point to then
>>> downgrade that to a read-lock. The access should be finished by that
>>> point.
>>>
>>
>> I was planning to take memory_lock read-lock at the beginning of
>> vfio_pci_config_rw() and release the same just before returning from
>> this function. If I don't downgrade it back to read-lock, then the
>> release in the end will be called for the lock which has not taken.
>> Also, user can specify count to any number of bytes and then the
>> vfio_config_do_rw() will be invoked multiple times and then in
>> the second call, it will be without lock.
>
> Ok, yes, I can imagine how it might result in a cleaner exit path to do
> a downgrade_write().
>
>>>> Also, what about IOCTLs. How can I take and release memory_lock for
>>>> ioctl. is it okay to go with Patch 7 where we call
>>>> pm_runtime_resume_and_get() before each ioctl or we need to do the
>>>> same low power check for ioctl also ?
>>>> In Later case, I am not sure how should I do the implementation so
>>>> that all other ioctl are covered from vfio core layer itself.
>>>
>>> Some ioctls clearly cannot occur while the device is in low power, such
>>> as resets and interrupt control, but even less obvious things like
>>> getting region info require device access. Migration also provides a
>>> channel to device access. Do we want to manage a list of ioctls that
>>> are allowed in low power, or do we only want to allow the ioctl to exit
>>> low power?
>>>
>>
>> In previous version of this patch, you mentioned that maintaining the
>> safe ioctl list will be tough to maintain. So, currently we wanted to
>> allow the ioctl for low power exit.
>
> Yes, I'm still conflicted in how that would work.
>
>>> I'm also still curious how we're going to handle devices that cannot
>>> return to low power such as the self-refresh mode on the GPU. We can
>>> potentially prevent any wake-ups from the vfio device interface, but
>>> that doesn't preclude a wake-up via an external lspci. I think we need
>>> to understand how we're going to handle such devices before we can
>>> really complete the design. AIUI, we cannot disable the self-refresh
>>> sleep mode without imposing unreasonable latency and memory
>>> requirements on the guest and we cannot retrigger the self-refresh
>>> low-power mode without non-trivial device specific code. Thanks,
>>>
>>> Alex
>>>
>>
>> I am working on adding support to notify guest through virtual PME
>> whenever there is any wake-up triggered by the host and the guest has
>> already put the device into runtime suspended state. This virtual PME
>> will be similar to physical PME. Normally, if PCI device need power
>> management transition, then it sends PME event which will be
>> ultimately handled by host OS. In virtual PME case, if host need power
>> management transition, then it sends event to guest and then guest OS
>> handles these virtual PME events. Following is summary:
>>
>> 1. Add the support for one more event like VFIO_PCI_ERR_IRQ_INDEX
>> named VFIO_PCI_PME_IRQ_INDEX and add the required code for this
>> virtual PME event.
>>
>> 2. From the guest side, when the PME_IRQ is enabled then we will
>> set event_fd for PME.
>>
>> 3. In the vfio driver, the PME support bits are already
>> virtualized and currently set to 0. We can set PME capability support
>> for D3cold so that in guest, it looks like
>>
>> Capabilities: [60] Power Management version 3
>> Flags: PMEClk- DSI- D1- D2- AuxCurrent=0mA
>> PME(D0-,D1-,D2-,D3hot-,D3cold+)
>>
>> 4. From the guest side, it can do PME enable (PME_En bit in Power
>> Management Control/Status Register) which will be again virtualized.
>>
>> 5. When host gets request for resuming the device other than from
>> low power ioctl, then device pm usage count will be incremented, the
>> PME status (PME_Status bit in Power Management Control/Status Register)
>> will be set and then we can do the event_fd signal.
>>
>> 6. In the PCIe, the PME events will be handled by root port. For
>> using low power D3cold feature, it is required to create virtual root
>> port in hypervisor side and when hypervisor receives this PME event,
>> then it can send virtual interrupt to root port.
>>
>> 7. If we take example of Linux kernel, then pcie_pme_irq() will
>> handle this and then do the runtime resume on the guest side. Also, it
>> will clear the PME status bit here. Then guest can put the device
>> again into suspended state.
>>
>> 8. I did prototype changes in QEMU for above logic and was getting wake-up
>> in the guest whenever I do lspci on the host side.
>>
>> 9. Since currently only nvidia GPU has this limitation to require
>> driver interaction each time before going into D3cold so we can allow
>> the reentry for other device. We can have nvidia vendor (along with
>> VGA/3D controller class code). In future, if any other device also has
>> similar requirement then we can update this list. For other device
>> host can put the device into D3cold in case of any wake-up.
>>
>> 10. In the vfio driver, we can put all these restriction for
>> enabling PME and return error if user tries to make low power entry
>> ioctl without enabling the PME related things.
>>
>> 11. The virtual PME can help in handling physical PME also for all
>> the devices. The PME logic is not dependent upon nvidia GPU
>> restriction. If virtual PME is enabled by hypervisor, then when
>> physical PME wakes the device, then it will resume on the guest side
>> also.
>
> So if host accesses through things like lspci are going to wake the
> device and we can't prevent that, and the solution to that is to notify
> the guest to put the device back to low power, then it seems a lot less
> important to try to prevent the user from waking the device through
> random accesses. In that context, maybe we do simply wrap all accesses
> with pm_runtime_get/put() put calls, which eliminates the problem of
> maintaining a list of safe ioctls in low power.
>

So wrap all access with pm_runtime_get()/put() will only be applicable
for IOCTLs. Correct ?
For config space, we can go with the approach discussed earlier in which
we return error ?

> I'd probably argue that whether to allow the kernel to put the device
> back to low power directly is a policy decision and should therefore be
> directed by userspace. For example the low power entry ioctl would
> have a flag to indicate the desired behavior and QEMU might have an
> on/off/[auto] vfio-pci device option which allows configuration of that
> behavior. The default auto policy might direct for automatic low-power
> re-entry except for NVIDIA VGA/3D class codes and other devices we
> discover that need it. This lets us have an immediate workaround for
> devices requiring guest support without a new kernel.
>

Yes. That is better option.
I will do the changes.

> This PME notification to the guest is really something that needs to be
> part of the base specification for user managed low power access due to
> these sorts of design decisions. Thanks,
>
> Alex
>

Yes. I will include this in my next patch series.

Regards,
Abhishek