[PATCH] vfio-mdev: support mediated device creation in kernel

[PATCH] vfio-mdev: support mediated device creation in kernel

[Yonghyun Hwang]

To enable a mediated device, a device driver registers its device to VFIO
MDev framework. Once the mediated device gets enabled, UUID gets fed onto
the sysfs attribute, "create", to create the mediated device. This
additional step happens after boot-up gets complete. If the driver knows
how many mediated devices need to be created during probing time, the
additional step becomes cumbersome. This commit implements a new function
to allow the driver to create a mediated device in kernel.

Signed-off-by: Yonghyun Hwang <yonghyun@google.com>
---
 drivers/vfio/mdev/mdev_core.c | 45 +++++++++++++++++++++++++++++++++++
 include/linux/mdev.h          |  3 +++
 2 files changed, 48 insertions(+)

diff --git a/drivers/vfio/mdev/mdev_core.c b/drivers/vfio/mdev/mdev_core.c
index b558d4cfd082..a6d32516de42 100644
--- a/drivers/vfio/mdev/mdev_core.c
+++ b/drivers/vfio/mdev/mdev_core.c
@@ -350,6 +350,51 @@ int mdev_device_create(struct kobject *kobj,
 	return ret;
 }
 
+/*
+ * mdev_create_device : Create a mdev device
+ * @dev: device structure representing parent device.
+ * @uuid: uuid char string for a mdev device.
+ * @group: index to supported type groups for a mdev device.
+ *
+ * Create a mdev device in kernel.
+ * Returns a negative value on error, otherwise 0.
+ */
+int mdev_create_device(struct device *dev,
+			const char *uuid, int group)
+{
+	struct mdev_parent *parent = NULL;
+	struct mdev_type *type = NULL;
+	guid_t guid;
+	int i = 1;
+	int ret;
+
+	ret = guid_parse(uuid, &guid);
+	if (ret) {
+		dev_err(dev, "Failed to parse UUID");
+		return ret;
+	}
+
+	parent = __find_parent_device(dev);
+	if (!parent) {
+		dev_err(dev, "Failed to find parent mdev device");
+		return -ENODEV;
+	}
+
+	list_for_each_entry(type, &parent->type_list, next) {
+		if (i == group)
+			break;
+		i++;
+	}
+
+	if (!type || i != group) {
+		dev_err(dev, "Failed to find mdev device");
+		return -ENODEV;
+	}
+
+	return mdev_device_create(&type->kobj, parent->dev, &guid);
+}
+EXPORT_SYMBOL(mdev_create_device);
+
 int mdev_device_remove(struct device *dev)
 {
 	struct mdev_device *mdev, *tmp;
diff --git a/include/linux/mdev.h b/include/linux/mdev.h
index 0ce30ca78db0..b66f67998916 100644
--- a/include/linux/mdev.h
+++ b/include/linux/mdev.h
@@ -145,4 +145,7 @@ struct device *mdev_parent_dev(struct mdev_device *mdev);
 struct device *mdev_dev(struct mdev_device *mdev);
 struct mdev_device *mdev_from_dev(struct device *dev);
 
+extern int mdev_create_device(struct device *dev,
+			const char *uuid, int group_idx);
+
 #endif /* MDEV_H */
-- 
2.25.1.696.g5e7596f4ac-goog

Re: [PATCH] vfio-mdev: support mediated device creation in kernel

[Alex Williamson]

On Fri, 20 Mar 2020 10:59:10 -0700
Yonghyun Hwang <yonghyun@google.com> wrote:

> To enable a mediated device, a device driver registers its device to VFIO
> MDev framework. Once the mediated device gets enabled, UUID gets fed onto
> the sysfs attribute, "create", to create the mediated device. This
> additional step happens after boot-up gets complete. If the driver knows
> how many mediated devices need to be created during probing time, the
> additional step becomes cumbersome. This commit implements a new function
> to allow the driver to create a mediated device in kernel.

But pre-creating mdev devices seems like a policy decision.  Why can't
userspace make such a policy decision, and do so with persistent uuids,
via something like mdevctl?  Thanks,

Alex

 
> Signed-off-by: Yonghyun Hwang <yonghyun@google.com>
> ---
>  drivers/vfio/mdev/mdev_core.c | 45 +++++++++++++++++++++++++++++++++++
>  include/linux/mdev.h          |  3 +++
>  2 files changed, 48 insertions(+)
> 
> diff --git a/drivers/vfio/mdev/mdev_core.c b/drivers/vfio/mdev/mdev_core.c
> index b558d4cfd082..a6d32516de42 100644
> --- a/drivers/vfio/mdev/mdev_core.c
> +++ b/drivers/vfio/mdev/mdev_core.c
> @@ -350,6 +350,51 @@ int mdev_device_create(struct kobject *kobj,
>  	return ret;
>  }
>  
> +/*
> + * mdev_create_device : Create a mdev device
> + * @dev: device structure representing parent device.
> + * @uuid: uuid char string for a mdev device.
> + * @group: index to supported type groups for a mdev device.
> + *
> + * Create a mdev device in kernel.
> + * Returns a negative value on error, otherwise 0.
> + */
> +int mdev_create_device(struct device *dev,
> +			const char *uuid, int group)
> +{
> +	struct mdev_parent *parent = NULL;
> +	struct mdev_type *type = NULL;
> +	guid_t guid;
> +	int i = 1;
> +	int ret;
> +
> +	ret = guid_parse(uuid, &guid);
> +	if (ret) {
> +		dev_err(dev, "Failed to parse UUID");
> +		return ret;
> +	}
> +
> +	parent = __find_parent_device(dev);
> +	if (!parent) {
> +		dev_err(dev, "Failed to find parent mdev device");
> +		return -ENODEV;
> +	}
> +
> +	list_for_each_entry(type, &parent->type_list, next) {
> +		if (i == group)
> +			break;
> +		i++;
> +	}
> +
> +	if (!type || i != group) {
> +		dev_err(dev, "Failed to find mdev device");
> +		return -ENODEV;
> +	}
> +
> +	return mdev_device_create(&type->kobj, parent->dev, &guid);
> +}
> +EXPORT_SYMBOL(mdev_create_device);
> +
>  int mdev_device_remove(struct device *dev)
>  {
>  	struct mdev_device *mdev, *tmp;
> diff --git a/include/linux/mdev.h b/include/linux/mdev.h
> index 0ce30ca78db0..b66f67998916 100644
> --- a/include/linux/mdev.h
> +++ b/include/linux/mdev.h
> @@ -145,4 +145,7 @@ struct device *mdev_parent_dev(struct mdev_device *mdev);
>  struct device *mdev_dev(struct mdev_device *mdev);
>  struct mdev_device *mdev_from_dev(struct device *dev);
>  
> +extern int mdev_create_device(struct device *dev,
> +			const char *uuid, int group_idx);
> +
>  #endif /* MDEV_H */

Re: [PATCH] vfio-mdev: support mediated device creation in kernel

[Yonghyun Hwang]

On Fri, Mar 20, 2020 at 11:34 AM Alex Williamson
<alex.williamson@redhat.com> wrote:
>
> On Fri, 20 Mar 2020 10:59:10 -0700
> Yonghyun Hwang <yonghyun@google.com> wrote:
>
> > To enable a mediated device, a device driver registers its device to VFIO
> > MDev framework. Once the mediated device gets enabled, UUID gets fed onto
> > the sysfs attribute, "create", to create the mediated device. This
> > additional step happens after boot-up gets complete. If the driver knows
> > how many mediated devices need to be created during probing time, the
> > additional step becomes cumbersome. This commit implements a new function
> > to allow the driver to create a mediated device in kernel.
>
> But pre-creating mdev devices seems like a policy decision.  Why can't
> userspace make such a policy decision, and do so with persistent uuids,
> via something like mdevctl?  Thanks,
>
> Alex

Yep, it can be viewed as the policy decision and userspace can make
the decision. However, it would be handy and plausible, if a device
driver can pre-create "fixed or default" # of mdev devices, while
allowing the device policy to come into play after bootup gets
complete. Without this patch, a device driver should release the
policy and the policy should be aligned with the driver, which would
be cumbersome (sometimes painful) in a cloud environment. My use case
with mdev is to enable a subset of vfio-pci features without losing my
device driver.

Thank you,
Yonghyun


>
>
> > Signed-off-by: Yonghyun Hwang <yonghyun@google.com>
> > ---
> >  drivers/vfio/mdev/mdev_core.c | 45 +++++++++++++++++++++++++++++++++++
> >  include/linux/mdev.h          |  3 +++
> >  2 files changed, 48 insertions(+)
> >
> > diff --git a/drivers/vfio/mdev/mdev_core.c b/drivers/vfio/mdev/mdev_core.c
> > index b558d4cfd082..a6d32516de42 100644
> > --- a/drivers/vfio/mdev/mdev_core.c
> > +++ b/drivers/vfio/mdev/mdev_core.c
> > @@ -350,6 +350,51 @@ int mdev_device_create(struct kobject *kobj,
> >       return ret;
> >  }
> >
> > +/*
> > + * mdev_create_device : Create a mdev device
> > + * @dev: device structure representing parent device.
> > + * @uuid: uuid char string for a mdev device.
> > + * @group: index to supported type groups for a mdev device.
> > + *
> > + * Create a mdev device in kernel.
> > + * Returns a negative value on error, otherwise 0.
> > + */
> > +int mdev_create_device(struct device *dev,
> > +                     const char *uuid, int group)
> > +{
> > +     struct mdev_parent *parent = NULL;
> > +     struct mdev_type *type = NULL;
> > +     guid_t guid;
> > +     int i = 1;
> > +     int ret;
> > +
> > +     ret = guid_parse(uuid, &guid);
> > +     if (ret) {
> > +             dev_err(dev, "Failed to parse UUID");
> > +             return ret;
> > +     }
> > +
> > +     parent = __find_parent_device(dev);
> > +     if (!parent) {
> > +             dev_err(dev, "Failed to find parent mdev device");
> > +             return -ENODEV;
> > +     }
> > +
> > +     list_for_each_entry(type, &parent->type_list, next) {
> > +             if (i == group)
> > +                     break;
> > +             i++;
> > +     }
> > +
> > +     if (!type || i != group) {
> > +             dev_err(dev, "Failed to find mdev device");
> > +             return -ENODEV;
> > +     }
> > +
> > +     return mdev_device_create(&type->kobj, parent->dev, &guid);
> > +}
> > +EXPORT_SYMBOL(mdev_create_device);
> > +
> >  int mdev_device_remove(struct device *dev)
> >  {
> >       struct mdev_device *mdev, *tmp;
> > diff --git a/include/linux/mdev.h b/include/linux/mdev.h
> > index 0ce30ca78db0..b66f67998916 100644
> > --- a/include/linux/mdev.h
> > +++ b/include/linux/mdev.h
> > @@ -145,4 +145,7 @@ struct device *mdev_parent_dev(struct mdev_device *mdev);
> >  struct device *mdev_dev(struct mdev_device *mdev);
> >  struct mdev_device *mdev_from_dev(struct device *dev);
> >
> > +extern int mdev_create_device(struct device *dev,
> > +                     const char *uuid, int group_idx);
> > +
> >  #endif /* MDEV_H */
>

Re: [PATCH] vfio-mdev: support mediated device creation in kernel

[Alex Williamson]

On Fri, 20 Mar 2020 13:46:04 -0700
Yonghyun Hwang <yonghyun@google.com> wrote:

> On Fri, Mar 20, 2020 at 11:34 AM Alex Williamson
> <alex.williamson@redhat.com> wrote:
> >
> > On Fri, 20 Mar 2020 10:59:10 -0700
> > Yonghyun Hwang <yonghyun@google.com> wrote:
> >  
> > > To enable a mediated device, a device driver registers its device to VFIO
> > > MDev framework. Once the mediated device gets enabled, UUID gets fed onto
> > > the sysfs attribute, "create", to create the mediated device. This
> > > additional step happens after boot-up gets complete. If the driver knows
> > > how many mediated devices need to be created during probing time, the
> > > additional step becomes cumbersome. This commit implements a new function
> > > to allow the driver to create a mediated device in kernel.  
> >
> > But pre-creating mdev devices seems like a policy decision.  Why can't
> > userspace make such a policy decision, and do so with persistent uuids,
> > via something like mdevctl?  Thanks,
> >
> > Alex  
> 
> Yep, it can be viewed as the policy decision and userspace can make
> the decision. However, it would be handy and plausible, if a device
> driver can pre-create "fixed or default" # of mdev devices, while
> allowing the device policy to come into play after bootup gets
> complete. Without this patch, a device driver should release the
> policy and the policy should be aligned with the driver, which would
> be cumbersome (sometimes painful) in a cloud environment. My use case
> with mdev is to enable a subset of vfio-pci features without losing my
> device driver.

Does this last comment suggest the parent device is not being
multiplexed through mdev, but only mediated?  If so, would something
like Yan's vendor-ops approach[1] be better?  Without a multiplexed
device, the lifecycle management of an mdev device doesn't make a lot
of sense, and I wonder if that's what you're trying to bypass here.
Even SR-IOV devices have moved to userspace enablement with most module
options to enable a default number of VFs being deprecated.  I do see
that that transition left a gap, but I'm not sure that heading in the
opposite direction with mdevs is a good idea either.  Thanks,

Alex

[1]https://lore.kernel.org/kvm/20200131020803.27519-1-yan.y.zhao@intel.com/


> > > Signed-off-by: Yonghyun Hwang <yonghyun@google.com>
> > > ---
> > >  drivers/vfio/mdev/mdev_core.c | 45 +++++++++++++++++++++++++++++++++++
> > >  include/linux/mdev.h          |  3 +++
> > >  2 files changed, 48 insertions(+)
> > >
> > > diff --git a/drivers/vfio/mdev/mdev_core.c b/drivers/vfio/mdev/mdev_core.c
> > > index b558d4cfd082..a6d32516de42 100644
> > > --- a/drivers/vfio/mdev/mdev_core.c
> > > +++ b/drivers/vfio/mdev/mdev_core.c
> > > @@ -350,6 +350,51 @@ int mdev_device_create(struct kobject *kobj,
> > >       return ret;
> > >  }
> > >
> > > +/*
> > > + * mdev_create_device : Create a mdev device
> > > + * @dev: device structure representing parent device.
> > > + * @uuid: uuid char string for a mdev device.
> > > + * @group: index to supported type groups for a mdev device.
> > > + *
> > > + * Create a mdev device in kernel.
> > > + * Returns a negative value on error, otherwise 0.
> > > + */
> > > +int mdev_create_device(struct device *dev,
> > > +                     const char *uuid, int group)
> > > +{
> > > +     struct mdev_parent *parent = NULL;
> > > +     struct mdev_type *type = NULL;
> > > +     guid_t guid;
> > > +     int i = 1;
> > > +     int ret;
> > > +
> > > +     ret = guid_parse(uuid, &guid);
> > > +     if (ret) {
> > > +             dev_err(dev, "Failed to parse UUID");
> > > +             return ret;
> > > +     }
> > > +
> > > +     parent = __find_parent_device(dev);
> > > +     if (!parent) {
> > > +             dev_err(dev, "Failed to find parent mdev device");
> > > +             return -ENODEV;
> > > +     }
> > > +
> > > +     list_for_each_entry(type, &parent->type_list, next) {
> > > +             if (i == group)
> > > +                     break;
> > > +             i++;
> > > +     }
> > > +
> > > +     if (!type || i != group) {
> > > +             dev_err(dev, "Failed to find mdev device");
> > > +             return -ENODEV;
> > > +     }
> > > +
> > > +     return mdev_device_create(&type->kobj, parent->dev, &guid);
> > > +}
> > > +EXPORT_SYMBOL(mdev_create_device);
> > > +
> > >  int mdev_device_remove(struct device *dev)
> > >  {
> > >       struct mdev_device *mdev, *tmp;
> > > diff --git a/include/linux/mdev.h b/include/linux/mdev.h
> > > index 0ce30ca78db0..b66f67998916 100644
> > > --- a/include/linux/mdev.h
> > > +++ b/include/linux/mdev.h
> > > @@ -145,4 +145,7 @@ struct device *mdev_parent_dev(struct mdev_device *mdev);
> > >  struct device *mdev_dev(struct mdev_device *mdev);
> > >  struct mdev_device *mdev_from_dev(struct device *dev);
> > >
> > > +extern int mdev_create_device(struct device *dev,
> > > +                     const char *uuid, int group_idx);
> > > +
> > >  #endif /* MDEV_H */  
> >  
> 

Re: [PATCH] vfio-mdev: support mediated device creation in kernel

[Yonghyun Hwang]

If my patch is not aligned with the direction, it results in a
maintenance burden for the kernel. Thanks for sharing the direction
and  Yan's vendor-ops approach[1].

Thank you,
Yonghyun

On Fri, Mar 20, 2020 at 1:59 PM Alex Williamson
<alex.williamson@redhat.com> wrote:
>
> On Fri, 20 Mar 2020 13:46:04 -0700
> Yonghyun Hwang <yonghyun@google.com> wrote:
>
> > On Fri, Mar 20, 2020 at 11:34 AM Alex Williamson
> > <alex.williamson@redhat.com> wrote:
> > >
> > > On Fri, 20 Mar 2020 10:59:10 -0700
> > > Yonghyun Hwang <yonghyun@google.com> wrote:
> > >
> > > > To enable a mediated device, a device driver registers its device to VFIO
> > > > MDev framework. Once the mediated device gets enabled, UUID gets fed onto
> > > > the sysfs attribute, "create", to create the mediated device. This
> > > > additional step happens after boot-up gets complete. If the driver knows
> > > > how many mediated devices need to be created during probing time, the
> > > > additional step becomes cumbersome. This commit implements a new function
> > > > to allow the driver to create a mediated device in kernel.
> > >
> > > But pre-creating mdev devices seems like a policy decision.  Why can't
> > > userspace make such a policy decision, and do so with persistent uuids,
> > > via something like mdevctl?  Thanks,
> > >
> > > Alex
> >
> > Yep, it can be viewed as the policy decision and userspace can make
> > the decision. However, it would be handy and plausible, if a device
> > driver can pre-create "fixed or default" # of mdev devices, while
> > allowing the device policy to come into play after bootup gets
> > complete. Without this patch, a device driver should release the
> > policy and the policy should be aligned with the driver, which would
> > be cumbersome (sometimes painful) in a cloud environment. My use case
> > with mdev is to enable a subset of vfio-pci features without losing my
> > device driver.
>
> Does this last comment suggest the parent device is not being
> multiplexed through mdev, but only mediated?  If so, would something
> like Yan's vendor-ops approach[1] be better?  Without a multiplexed
> device, the lifecycle management of an mdev device doesn't make a lot
> of sense, and I wonder if that's what you're trying to bypass here.
> Even SR-IOV devices have moved to userspace enablement with most module
> options to enable a default number of VFs being deprecated.  I do see
> that that transition left a gap, but I'm not sure that heading in the
> opposite direction with mdevs is a good idea either.  Thanks,
>
> Alex
>
> [1]https://lore.kernel.org/kvm/20200131020803.27519-1-yan.y.zhao@intel.com/
>
>
> > > > Signed-off-by: Yonghyun Hwang <yonghyun@google.com>
> > > > ---
> > > >  drivers/vfio/mdev/mdev_core.c | 45 +++++++++++++++++++++++++++++++++++
> > > >  include/linux/mdev.h          |  3 +++
> > > >  2 files changed, 48 insertions(+)
> > > >
> > > > diff --git a/drivers/vfio/mdev/mdev_core.c b/drivers/vfio/mdev/mdev_core.c
> > > > index b558d4cfd082..a6d32516de42 100644
> > > > --- a/drivers/vfio/mdev/mdev_core.c
> > > > +++ b/drivers/vfio/mdev/mdev_core.c
> > > > @@ -350,6 +350,51 @@ int mdev_device_create(struct kobject *kobj,
> > > >       return ret;
> > > >  }
> > > >
> > > > +/*
> > > > + * mdev_create_device : Create a mdev device
> > > > + * @dev: device structure representing parent device.
> > > > + * @uuid: uuid char string for a mdev device.
> > > > + * @group: index to supported type groups for a mdev device.
> > > > + *
> > > > + * Create a mdev device in kernel.
> > > > + * Returns a negative value on error, otherwise 0.
> > > > + */
> > > > +int mdev_create_device(struct device *dev,
> > > > +                     const char *uuid, int group)
> > > > +{
> > > > +     struct mdev_parent *parent = NULL;
> > > > +     struct mdev_type *type = NULL;
> > > > +     guid_t guid;
> > > > +     int i = 1;
> > > > +     int ret;
> > > > +
> > > > +     ret = guid_parse(uuid, &guid);
> > > > +     if (ret) {
> > > > +             dev_err(dev, "Failed to parse UUID");
> > > > +             return ret;
> > > > +     }
> > > > +
> > > > +     parent = __find_parent_device(dev);
> > > > +     if (!parent) {
> > > > +             dev_err(dev, "Failed to find parent mdev device");
> > > > +             return -ENODEV;
> > > > +     }
> > > > +
> > > > +     list_for_each_entry(type, &parent->type_list, next) {
> > > > +             if (i == group)
> > > > +                     break;
> > > > +             i++;
> > > > +     }
> > > > +
> > > > +     if (!type || i != group) {
> > > > +             dev_err(dev, "Failed to find mdev device");
> > > > +             return -ENODEV;
> > > > +     }
> > > > +
> > > > +     return mdev_device_create(&type->kobj, parent->dev, &guid);
> > > > +}
> > > > +EXPORT_SYMBOL(mdev_create_device);
> > > > +
> > > >  int mdev_device_remove(struct device *dev)
> > > >  {
> > > >       struct mdev_device *mdev, *tmp;
> > > > diff --git a/include/linux/mdev.h b/include/linux/mdev.h
> > > > index 0ce30ca78db0..b66f67998916 100644
> > > > --- a/include/linux/mdev.h
> > > > +++ b/include/linux/mdev.h
> > > > @@ -145,4 +145,7 @@ struct device *mdev_parent_dev(struct mdev_device *mdev);
> > > >  struct device *mdev_dev(struct mdev_device *mdev);
> > > >  struct mdev_device *mdev_from_dev(struct device *dev);
> > > >
> > > > +extern int mdev_create_device(struct device *dev,
> > > > +                     const char *uuid, int group_idx);
> > > > +
> > > >  #endif /* MDEV_H */
> > >
> >
>

Re: [PATCH] vfio-mdev: support mediated device creation in kernel

[Christoph Hellwig]

On Fri, Mar 20, 2020 at 10:59:10AM -0700, Yonghyun Hwang wrote:
> To enable a mediated device, a device driver registers its device to VFIO
> MDev framework. Once the mediated device gets enabled, UUID gets fed onto
> the sysfs attribute, "create", to create the mediated device. This
> additional step happens after boot-up gets complete. If the driver knows
> how many mediated devices need to be created during probing time, the
> additional step becomes cumbersome. This commit implements a new function
> to allow the driver to create a mediated device in kernel.

Please send this along with your proposed user so that we can understand
the use.  Without that new exports have no chance of going in anyway.

Re: [PATCH] vfio-mdev: support mediated device creation in kernel

[Yonghyun Hwang]

On Mon, Mar 23, 2020 at 4:14 AM Christoph Hellwig <hch@infradead.org> wrote:
>
> On Fri, Mar 20, 2020 at 10:59:10AM -0700, Yonghyun Hwang wrote:
> > To enable a mediated device, a device driver registers its device to VFIO
> > MDev framework. Once the mediated device gets enabled, UUID gets fed onto
> > the sysfs attribute, "create", to create the mediated device. This
> > additional step happens after boot-up gets complete. If the driver knows
> > how many mediated devices need to be created during probing time, the
> > additional step becomes cumbersome. This commit implements a new function
> > to allow the driver to create a mediated device in kernel.
>
> Please send this along with your proposed user so that we can understand
> the use.  Without that new exports have no chance of going in anyway.

My driver is still under development. Do you recommend me to implement
an example code for the new exports and re-submit the commit?

Thank you,
Yonghyun

Re: [PATCH] vfio-mdev: support mediated device creation in kernel

[Christoph Hellwig]

On Mon, Mar 23, 2020 at 02:33:11PM -0700, Yonghyun Hwang wrote:
> On Mon, Mar 23, 2020 at 4:14 AM Christoph Hellwig <hch@infradead.org> wrote:
> >
> > On Fri, Mar 20, 2020 at 10:59:10AM -0700, Yonghyun Hwang wrote:
> > > To enable a mediated device, a device driver registers its device to VFIO
> > > MDev framework. Once the mediated device gets enabled, UUID gets fed onto
> > > the sysfs attribute, "create", to create the mediated device. This
> > > additional step happens after boot-up gets complete. If the driver knows
> > > how many mediated devices need to be created during probing time, the
> > > additional step becomes cumbersome. This commit implements a new function
> > > to allow the driver to create a mediated device in kernel.
> >
> > Please send this along with your proposed user so that we can understand
> > the use.  Without that new exports have no chance of going in anyway.
> 
> My driver is still under development. Do you recommend me to implement
> an example code for the new exports and re-submit the commit?

Hell no.  The point is that we don't add new APIs unless we have
actual users (not example code!).  And as Alex mentioned the use case
is rather questionable anyway, so without a user that actually shows a
good use case which would remove those doubts it is a complete no-go.

Re: [PATCH] vfio-mdev: support mediated device creation in kernel

[Yonghyun Hwang]

On Thu, Mar 26, 2020 at 2:38 AM Christoph Hellwig <hch@infradead.org> wrote:
>
> On Mon, Mar 23, 2020 at 02:33:11PM -0700, Yonghyun Hwang wrote:
> > On Mon, Mar 23, 2020 at 4:14 AM Christoph Hellwig <hch@infradead.org> wrote:
> > >
> > > On Fri, Mar 20, 2020 at 10:59:10AM -0700, Yonghyun Hwang wrote:
> > > > To enable a mediated device, a device driver registers its device to VFIO
> > > > MDev framework. Once the mediated device gets enabled, UUID gets fed onto
> > > > the sysfs attribute, "create", to create the mediated device. This
> > > > additional step happens after boot-up gets complete. If the driver knows
> > > > how many mediated devices need to be created during probing time, the
> > > > additional step becomes cumbersome. This commit implements a new function
> > > > to allow the driver to create a mediated device in kernel.
> > >
> > > Please send this along with your proposed user so that we can understand
> > > the use.  Without that new exports have no chance of going in anyway.
> >
> > My driver is still under development. Do you recommend me to implement
> > an example code for the new exports and re-submit the commit?
>
> Hell no.  The point is that we don't add new APIs unless we have
> actual users (not example code!).  And as Alex mentioned the use case
> is rather questionable anyway, so without a user that actually shows a
> good use case which would remove those doubts it is a complete no-go.

I see. Thank you for your clarification. (I got confused with the use
case.) I hope you can understand email communication results in some
confusion from time to time, which is especially true for kernel
newbie like me.