linux-kernel - Re: [PATCH v4 2/4] vfio/pci: Change the PF power state to D0 before enabling VFs

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Message-ID: <1df0a2f1-928a-034a-b5b5-1b24b09ce1d5@nvidia.com>
Date:   Wed, 18 May 2022 15:26:37 +0530
From:   Abhishek Sahu <abhsahu@...dia.com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     Cornelia Huck <cohuck@...hat.com>,
        Yishai Hadas <yishaih@...dia.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Shameer Kolothum <shameerali.kolothum.thodi@...wei.com>,
        Kevin Tian <kevin.tian@...el.com>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Max Gurtovoy <mgurtovoy@...dia.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-pm@...r.kernel.org, linux-pci@...r.kernel.org
Subject: Re: [PATCH v4 2/4] vfio/pci: Change the PF power state to D0 before
 enabling VFs

On 5/17/2022 11:57 PM, Alex Williamson wrote:
> On Tue, 17 May 2022 15:32:17 +0530
> Abhishek Sahu <abhsahu@...dia.com> wrote:
> 
>> According to [PCIe v5 9.6.2] for PF Device Power Management States
>>
>>  "The PF's power management state (D-state) has global impact on its
>>   associated VFs. If a VF does not implement the Power Management
>>   Capability, then it behaves as if it is in an equivalent
>>   power state of its associated PF.
>>
>>   If a VF implements the Power Management Capability, the Device behavior
>>   is undefined if the PF is placed in a lower power state than the VF.
>>   Software should avoid this situation by placing all VFs in lower power
>>   state before lowering their associated PF's power state."
>>
>> From the vfio driver side, user can enable SR-IOV when the PF is in D3hot
>> state. If VF does not implement the Power Management Capability, then
>> the VF will be actually in D3hot state and then the VF BAR access will
>> fail. If VF implements the Power Management Capability, then VF will
>> assume that its current power state is D0 when the PF is D3hot and
>> in this case, the behavior is undefined.
>>
>> To support PF power management, we need to create power management
>> dependency between PF and its VF's. The runtime power management support
>> may help with this where power management dependencies are supported
>> through device links. But till we have such support in place, we can
>> disallow the PF to go into low power state, if PF has VF enabled.
>> There can be a case, where user first enables the VF's and then
>> disables the VF's. If there is no user of PF, then the PF can put into
>> D3hot state again. But with this patch, the PF will still be in D0
>> state after disabling VF's since detecting this case inside
>> vfio_pci_core_sriov_configure() requires access to
>> struct vfio_device::open_count along with its locks. But the subsequent
>> patches related to runtime PM will handle this case since runtime PM
>> maintains its own usage count.
>>
>> Also, vfio_pci_core_sriov_configure() can be called at any time
>> (with and without vfio pci device user), so the power state change
>> needs to be protected with the required locks.
>>
>> Signed-off-by: Abhishek Sahu <abhsahu@...dia.com>
>> ---
>>  drivers/vfio/pci/vfio_pci_core.c | 11 +++++++++++
>>  1 file changed, 11 insertions(+)
>>
>> diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
>> index b9f222ca48cf..4fe9a4efc751 100644
>> --- a/drivers/vfio/pci/vfio_pci_core.c
>> +++ b/drivers/vfio/pci/vfio_pci_core.c
>> @@ -217,6 +217,10 @@ int vfio_pci_set_power_state(struct vfio_pci_core_device *vdev, pci_power_t stat
>>  	bool needs_restore = false, needs_save = false;
>>  	int ret;
>>  
>> +	/* Prevent changing power state for PFs with VFs enabled */
>> +	if (pci_num_vf(pdev) && state > PCI_D0)
>> +		return -EBUSY;
>> +
>>  	if (vdev->needs_pm_restore) {
>>  		if (pdev->current_state < PCI_D3hot && state >= PCI_D3hot) {
>>  			pci_save_state(pdev);
>> @@ -1960,6 +1964,13 @@ int vfio_pci_core_sriov_configure(struct vfio_pci_core_device *vdev,
>>  		}
>>  		list_add_tail(&vdev->sriov_pfs_item, &vfio_pci_sriov_pfs);
>>  		mutex_unlock(&vfio_pci_sriov_pfs_mutex);
>> +
>> +		/*
>> +		 * The PF power state should always be higher than the VF power
>> +		 * state. If PF is in the low power state, then change the
>> +		 * power state to D0 first before enabling SR-IOV.
>> +		 */
>> +		vfio_pci_lock_and_set_power_state(vdev, PCI_D0);
> 
> But we need to hold memory_lock across the next function or else
> userspace could race a write to the PM register to set D3 before
> pci_num_vf() can protect us.  Thanks,
> 
> Alex
> 

 Thanks Alex.
 Yes. We need to bring pci_enable_sriov() also to protect this race
 condition. I will update this in my next version.
 
 Regards,
 Abhishek

>>  		ret = pci_enable_sriov(pdev, nr_virtfn);
>>  		if (ret)
>>  			goto out_del;
>