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

Abhishek Sahu
2022-05-18T09:57:03Z

On 5/17/2022 11:57 PM, Alex Williamson wrote:
> On Tue, 17 May 2022 15:32:17 +0530
> Abhishek Sahu <abhsahu@nvidia.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@nvidia.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;
>