Re: [PATCH v4 3/6] vfio: Increment the runtime PM usage count during IOCTL call

[Abhishek Sahu <abhsahu@nvidia.com>]

On 7/6/2022 9:10 PM, Alex Williamson wrote:
> On Fri, 1 Jul 2022 16:38:11 +0530
> Abhishek Sahu <abhsahu@nvidia.com> wrote:
> 
>> The vfio-pci based driver will have runtime power management
>> support where the user can put the device into the low power state
>> and then PCI devices can go into the D3cold state. If the device is
>> in the low power state and the user issues any IOCTL, then the
>> device should be moved out of the low power state first. Once
>> the IOCTL is serviced, then it can go into the low power state again.
>> The runtime PM framework manages this with help of usage count.
>>
>> One option was to add the runtime PM related API's inside vfio-pci
>> driver but some IOCTL (like VFIO_DEVICE_FEATURE) can follow a
>> different path and more IOCTL can be added in the future. Also, the
>> runtime PM will be added for vfio-pci based drivers variant currently,
>> but the other VFIO based drivers can use the same in the
>> future. So, this patch adds the runtime calls runtime-related API in
>> the top-level IOCTL function itself.
>>
>> For the VFIO drivers which do not have runtime power management
>> support currently, the runtime PM API's won't be invoked. Only for
>> vfio-pci based drivers currently, the runtime PM API's will be invoked
>> to increment and decrement the usage count.
> 
> Variant drivers can easily opt-out of runtime pm support by performing
> a gratuitous pm-get in their device-open function.
>  

 Do I need to add this line in the commit message?
 
>> Taking this usage count incremented while servicing IOCTL will make
>> sure that the user won't put the device into low power state when any
>> other IOCTL is being serviced in parallel. Let's consider the
>> following scenario:
>>
>>  1. Some other IOCTL is called.
>>  2. The user has opened another device instance and called the power
>>     management IOCTL for the low power entry.
>>  3. The power management IOCTL moves the device into the low power state.
>>  4. The other IOCTL finishes.
>>
>> If we don't keep the usage count incremented then the device
>> access will happen between step 3 and 4 while the device has already
>> gone into the low power state.
>>
>> The runtime PM API's should not be invoked for
>> VFIO_DEVICE_FEATURE_POWER_MANAGEMENT since this IOCTL itself performs
>> the runtime power management entry and exit for the VFIO device.
> 
> I think the one-shot interface I proposed in the previous patch avoids
> the need for special handling for these feature ioctls.  Thanks,
> 

 Okay. So, for low power exit case (means feature GET ioctl in the
 updated case) also, we will trigger eventfd. Correct?

 Thanks,
 Abhishek
 
> Alex
>  
>> The pm_runtime_resume_and_get() will be the first call so its error
>> should not be propagated to user space directly. For example, if
>> pm_runtime_resume_and_get() can return -EINVAL for the cases where the
>> user has passed the correct argument. So the
>> pm_runtime_resume_and_get() errors have been masked behind -EIO.
>>
>> Signed-off-by: Abhishek Sahu <abhsahu@nvidia.com>
>> ---
>>  drivers/vfio/vfio.c | 82 ++++++++++++++++++++++++++++++++++++++++-----
>>  1 file changed, 74 insertions(+), 8 deletions(-)
>>
>> diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
>> index 61e71c1154be..61a8d9f7629a 100644
>> --- a/drivers/vfio/vfio.c
>> +++ b/drivers/vfio/vfio.c
>> @@ -32,6 +32,7 @@
>>  #include <linux/vfio.h>
>>  #include <linux/wait.h>
>>  #include <linux/sched/signal.h>
>> +#include <linux/pm_runtime.h>
>>  #include "vfio.h"
>>  
>>  #define DRIVER_VERSION	"0.3"
>> @@ -1333,6 +1334,39 @@ static const struct file_operations vfio_group_fops = {
>>  	.release	= vfio_group_fops_release,
>>  };
>>  
>> +/*
>> + * Wrapper around pm_runtime_resume_and_get().
>> + * Return error code on failure or 0 on success.
>> + */
>> +static inline int vfio_device_pm_runtime_get(struct vfio_device *device)
>> +{
>> +	struct device *dev = device->dev;
>> +
>> +	if (dev->driver && dev->driver->pm) {
>> +		int ret;
>> +
>> +		ret = pm_runtime_resume_and_get(dev);
>> +		if (ret < 0) {
>> +			dev_info_ratelimited(dev,
>> +				"vfio: runtime resume failed %d\n", ret);
>> +			return -EIO;
>> +		}
>> +	}
>> +
>> +	return 0;
>> +}
>> +
>> +/*
>> + * Wrapper around pm_runtime_put().
>> + */
>> +static inline void vfio_device_pm_runtime_put(struct vfio_device *device)
>> +{
>> +	struct device *dev = device->dev;
>> +
>> +	if (dev->driver && dev->driver->pm)
>> +		pm_runtime_put(dev);
>> +}
>> +
>>  /*
>>   * VFIO Device fd
>>   */
>> @@ -1607,6 +1641,8 @@ static int vfio_ioctl_device_feature(struct vfio_device *device,
>>  {
>>  	size_t minsz = offsetofend(struct vfio_device_feature, flags);
>>  	struct vfio_device_feature feature;
>> +	int ret = 0;
>> +	u16 feature_cmd;
>>  
>>  	if (copy_from_user(&feature, arg, minsz))
>>  		return -EFAULT;
>> @@ -1626,28 +1662,51 @@ static int vfio_ioctl_device_feature(struct vfio_device *device,
>>  	    (feature.flags & VFIO_DEVICE_FEATURE_GET))
>>  		return -EINVAL;
>>  
>> -	switch (feature.flags & VFIO_DEVICE_FEATURE_MASK) {
>> +	feature_cmd = feature.flags & VFIO_DEVICE_FEATURE_MASK;
>> +
>> +	/*
>> +	 * The VFIO_DEVICE_FEATURE_POWER_MANAGEMENT itself performs the runtime
>> +	 * power management entry and exit for the VFIO device, so the runtime
>> +	 * PM API's should not be called for this feature.
>> +	 */
>> +	if (feature_cmd != VFIO_DEVICE_FEATURE_POWER_MANAGEMENT) {
>> +		ret = vfio_device_pm_runtime_get(device);
>> +		if (ret)
>> +			return ret;
>> +	}
>> +
>> +	switch (feature_cmd) {
>>  	case VFIO_DEVICE_FEATURE_MIGRATION:
>> -		return vfio_ioctl_device_feature_migration(
>> +		ret = vfio_ioctl_device_feature_migration(
>>  			device, feature.flags, arg->data,
>>  			feature.argsz - minsz);
>> +		break;
>>  	case VFIO_DEVICE_FEATURE_MIG_DEVICE_STATE:
>> -		return vfio_ioctl_device_feature_mig_device_state(
>> +		ret = vfio_ioctl_device_feature_mig_device_state(
>>  			device, feature.flags, arg->data,
>>  			feature.argsz - minsz);
>> +		break;
>>  	default:
>>  		if (unlikely(!device->ops->device_feature))
>> -			return -EINVAL;
>> -		return device->ops->device_feature(device, feature.flags,
>> -						   arg->data,
>> -						   feature.argsz - minsz);
>> +			ret = -EINVAL;
>> +		else
>> +			ret = device->ops->device_feature(
>> +				device, feature.flags, arg->data,
>> +				feature.argsz - minsz);
>> +		break;
>>  	}
>> +
>> +	if (feature_cmd != VFIO_DEVICE_FEATURE_POWER_MANAGEMENT)
>> +		vfio_device_pm_runtime_put(device);
>> +
>> +	return ret;
>>  }
>>  
>>  static long vfio_device_fops_unl_ioctl(struct file *filep,
>>  				       unsigned int cmd, unsigned long arg)
>>  {
>>  	struct vfio_device *device = filep->private_data;
>> +	int ret;
>>  
>>  	switch (cmd) {
>>  	case VFIO_DEVICE_FEATURE:
>> @@ -1655,7 +1714,14 @@ static long vfio_device_fops_unl_ioctl(struct file *filep,
>>  	default:
>>  		if (unlikely(!device->ops->ioctl))
>>  			return -EINVAL;
>> -		return device->ops->ioctl(device, cmd, arg);
>> +
>> +		ret = vfio_device_pm_runtime_get(device);
>> +		if (ret)
>> +			return ret;
>> +
>> +		ret = device->ops->ioctl(device, cmd, arg);
>> +		vfio_device_pm_runtime_put(device);
>> +		return ret;
>>  	}
>>  }
>>  
>