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Date: Wed, 6 Jul 2022 09:40:07 -0600
From: Alex Williamson <alex.williamson@...hat.com>
To: Abhishek Sahu <abhsahu@...dia.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 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.
> 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,
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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