Re: [PATCH v4 1/6] vfio/pci: Mask INTx during runtime suspend

[Alex Williamson <alex.williamson@redhat.com>]

On Mon, 11 Jul 2022 14:48:34 +0530
Abhishek Sahu <abhsahu@nvidia.com> wrote:

> On 7/8/2022 9:15 PM, Alex Williamson wrote:
> > On Fri, 8 Jul 2022 14:51:30 +0530
> > Abhishek Sahu <abhsahu@nvidia.com> wrote:
> >   
> >> On 7/6/2022 9:09 PM, Alex Williamson wrote:  
> >>> On Fri, 1 Jul 2022 16:38:09 +0530
> >>> Abhishek Sahu <abhsahu@nvidia.com> wrote:
> >>>     
> >>>> This patch adds INTx handling during runtime suspend/resume.
> >>>> All the suspend/resume related code for the user to put the device
> >>>> into the low power state will be added in subsequent patches.
> >>>>
> >>>> The INTx are shared among devices. Whenever any INTx interrupt comes    
> >>>
> >>> "The INTx lines may be shared..."
> >>>     
> >>>> for the VFIO devices, then vfio_intx_handler() will be called for each
> >>>> device. Inside vfio_intx_handler(), it calls pci_check_and_mask_intx()    
> >>>
> >>> "...device sharing the interrupt."
> >>>     
> >>>> and checks if the interrupt has been generated for the current device.
> >>>> Now, if the device is already in the D3cold state, then the config space
> >>>> can not be read. Attempt to read config space in D3cold state can
> >>>> cause system unresponsiveness in a few systems. To prevent this, mask
> >>>> INTx in runtime suspend callback and unmask the same in runtime resume
> >>>> callback. If INTx has been already masked, then no handling is needed
> >>>> in runtime suspend/resume callbacks. 'pm_intx_masked' tracks this, and
> >>>> vfio_pci_intx_mask() has been updated to return true if INTx has been
> >>>> masked inside this function.
> >>>>
> >>>> For the runtime suspend which is triggered for the no user of VFIO
> >>>> device, the is_intx() will return false and these callbacks won't do
> >>>> anything.
> >>>>
> >>>> The MSI/MSI-X are not shared so similar handling should not be
> >>>> needed for MSI/MSI-X. vfio_msihandler() triggers eventfd_signal()
> >>>> without doing any device-specific config access. When the user performs
> >>>> any config access or IOCTL after receiving the eventfd notification,
> >>>> then the device will be moved to the D0 state first before
> >>>> servicing any request.
> >>>>
> >>>> Signed-off-by: Abhishek Sahu <abhsahu@nvidia.com>
> >>>> ---
> >>>>  drivers/vfio/pci/vfio_pci_core.c  | 37 +++++++++++++++++++++++++++----
> >>>>  drivers/vfio/pci/vfio_pci_intrs.c |  6 ++++-
> >>>>  include/linux/vfio_pci_core.h     |  3 ++-
> >>>>  3 files changed, 40 insertions(+), 6 deletions(-)
> >>>>
> >>>> diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
> >>>> index a0d69ddaf90d..5948d930449b 100644
> >>>> --- a/drivers/vfio/pci/vfio_pci_core.c
> >>>> +++ b/drivers/vfio/pci/vfio_pci_core.c
> >>>> @@ -259,16 +259,45 @@ int vfio_pci_set_power_state(struct vfio_pci_core_device *vdev, pci_power_t stat
> >>>>  	return ret;
> >>>>  }
> >>>>  
> >>>> +#ifdef CONFIG_PM
> >>>> +static int vfio_pci_core_runtime_suspend(struct device *dev)
> >>>> +{
> >>>> +	struct vfio_pci_core_device *vdev = dev_get_drvdata(dev);
> >>>> +
> >>>> +	/*
> >>>> +	 * If INTx is enabled, then mask INTx before going into the runtime
> >>>> +	 * suspended state and unmask the same in the runtime resume.
> >>>> +	 * If INTx has already been masked by the user, then
> >>>> +	 * vfio_pci_intx_mask() will return false and in that case, INTx
> >>>> +	 * should not be unmasked in the runtime resume.
> >>>> +	 */
> >>>> +	vdev->pm_intx_masked = (is_intx(vdev) && vfio_pci_intx_mask(vdev));
> >>>> +
> >>>> +	return 0;
> >>>> +}
> >>>> +
> >>>> +static int vfio_pci_core_runtime_resume(struct device *dev)
> >>>> +{
> >>>> +	struct vfio_pci_core_device *vdev = dev_get_drvdata(dev);
> >>>> +
> >>>> +	if (vdev->pm_intx_masked)
> >>>> +		vfio_pci_intx_unmask(vdev);
> >>>> +
> >>>> +	return 0;
> >>>> +}
> >>>> +#endif /* CONFIG_PM */
> >>>> +
> >>>>  /*
> >>>> - * The dev_pm_ops needs to be provided to make pci-driver runtime PM working,
> >>>> - * so use structure without any callbacks.
> >>>> - *
> >>>>   * The pci-driver core runtime PM routines always save the device state
> >>>>   * before going into suspended state. If the device is going into low power
> >>>>   * state with only with runtime PM ops, then no explicit handling is needed
> >>>>   * for the devices which have NoSoftRst-.
> >>>>   */
> >>>> -static const struct dev_pm_ops vfio_pci_core_pm_ops = { };
> >>>> +static const struct dev_pm_ops vfio_pci_core_pm_ops = {
> >>>> +	SET_RUNTIME_PM_OPS(vfio_pci_core_runtime_suspend,
> >>>> +			   vfio_pci_core_runtime_resume,
> >>>> +			   NULL)
> >>>> +};
> >>>>  
> >>>>  int vfio_pci_core_enable(struct vfio_pci_core_device *vdev)
> >>>>  {
> >>>> diff --git a/drivers/vfio/pci/vfio_pci_intrs.c b/drivers/vfio/pci/vfio_pci_intrs.c
> >>>> index 6069a11fb51a..1a37db99df48 100644
> >>>> --- a/drivers/vfio/pci/vfio_pci_intrs.c
> >>>> +++ b/drivers/vfio/pci/vfio_pci_intrs.c
> >>>> @@ -33,10 +33,12 @@ static void vfio_send_intx_eventfd(void *opaque, void *unused)
> >>>>  		eventfd_signal(vdev->ctx[0].trigger, 1);
> >>>>  }
> >>>>  
> >>>> -void vfio_pci_intx_mask(struct vfio_pci_core_device *vdev)
> >>>> +/* Returns true if INTx has been masked by this function. */
> >>>> +bool vfio_pci_intx_mask(struct vfio_pci_core_device *vdev)
> >>>>  {
> >>>>  	struct pci_dev *pdev = vdev->pdev;
> >>>>  	unsigned long flags;
> >>>> +	bool intx_masked = false;
> >>>>  
> >>>>  	spin_lock_irqsave(&vdev->irqlock, flags);
> >>>>  
> >>>> @@ -60,9 +62,11 @@ void vfio_pci_intx_mask(struct vfio_pci_core_device *vdev)
> >>>>  			disable_irq_nosync(pdev->irq);
> >>>>  
> >>>>  		vdev->ctx[0].masked = true;
> >>>> +		intx_masked = true;
> >>>>  	}
> >>>>  
> >>>>  	spin_unlock_irqrestore(&vdev->irqlock, flags);
> >>>> +	return intx_masked;
> >>>>  }    
> >>>
> >>>
> >>> There's certainly another path through this function that masks the
> >>> interrupt, which makes the definition of this return value a bit
> >>> confusing.    
> >>
> >>  For our case we should not hit that path. But we can return the
> >>  intx_masked true from that path as well to align return value.
> >>  
> >>> Wouldn't it be simpler not to overload the masked flag on
> >>> the interrupt context like this and instead set a new flag on the vdev
> >>> under irqlock to indicate the device is unable to generate interrupts.
> >>> The irq handler would add a test of this flag before any tests that
> >>> would access the device.  Thanks,
> >>>
> >>> Alex
> >>>      
> >>
> >>  We will set this flag inside runtime_suspend callback but the
> >>  device can be in non-D3cold state (For example, if user has
> >>  disabled d3cold explicitly by sysfs, the D3cold is not supported in
> >>  the platform, etc.). Also, in D3cold supported case, the device will
> >>  be in D0 till the PCI core moves the device into D3cold. In this case,
> >>  there is possibility that the device can generate an interrupt.
> >>  If we add check in the IRQ handler, then we won’t check and clear
> >>  the IRQ status, but the interrupt line will still be asserted
> >>  which can cause interrupt flooding.
> >>
> >>  This was the reason for disabling interrupt itself instead of
> >>  checking flag in the IRQ handler.  
> > 
> > Ok, maybe this is largely a clarification of the return value of
> > vfio_pci_intx_mask().  I think what you're looking for is whether the
> > context mask was changed, rather than whether the interrupt is masked,
> > which I think avoids the confusion regarding whether the first branch
> > should return true or false.  So the variable should be something like
> > "masked_changed" and the comment changed to "Returns true if the INTx
> > vfio_pci_irq_ctx.masked value is changed".
> >   
> 
>  Thanks Alex.
>  I will rename the variable and update the comment.
> 
> > Testing is_intx() outside of the irqlock is potentially racy, so do we
> > need to add the pm-get/put wrappers on ioctls first to avoid the
> > possibility that pm-suspend can race a SET_IRQS ioctl?  Thanks,
> > 
> > Alex
> >   
>  
>  Even after adding this patch, the runtime suspend will not be supported
>  for the device with users. It will be supported only after patch 4 in
>  this patch series. So with this patch, the pm-suspend can be called only
>  for the cases where vfio device has no user and there we should not see
>  the race condition.

We should also not see is_intx() true for unused devices.  Thanks,

Alex