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Date: Fri, 8 Jul 2022 15:13:16 +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 3/6] vfio: Increment the runtime PM usage count during
IOCTL call
On 7/6/2022 9:10 PM, Alex Williamson wrote:
> On Fri, 1 Jul 2022 16:38:11 +0530
> Abhishek Sahu <abhsahu@...dia.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@...dia.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;
>> }
>> }
>>
>
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