[libvirt] [RFC PATCH v1 1/1] Add attribute multiple_mdev_support for mdev type-id

[Tian, Kevin]

> From: Kirti Wankhede
> Sent: Thursday, September 27, 2018 2:22 PM
> 
> Generally a single instance of mdev device, a share of physical device, is
> assigned to user space application or a VM. There are cases when multiple
> instances of mdev devices of same or different types are required by User
> space application or VM. For example in case of vGPU, multiple mdev
> devices
> of type which represents whole GPU can be assigned to one instance of
> application or VM.
> 
> All types of mdev devices may not support assigning multiple mdev devices
> to a user space application. In that case vendor driver can fail open()
> call of mdev device. But there is no way to know User space application
> about the configuration supported by vendor driver.
> 
> To expose supported configuration, vendor driver should add
> 'multiple_mdev_support' attribute to type-id directory if vendor driver
> supports assigning multiple mdev devices of a particular type-id to one
> instance of user space application or a VM.
> 
> User space application should read if 'multiple_mdev_support' attibute is
> present in type-id directory of all mdev devices which are going to be
> used. If all read 1 then user space application can proceed with multiple
> mdev devices.
> 
> This is optional and readonly attribute.

I didn't get what is the exact problem from above description. Isn't it the
basic point behind mdev concept that parent driver can create multiple
mdev instances on a physical device? VFIO usually doesn't care whether
multiple devices (pci or mdev) are assigned to same process/VM or not.
Can you give some example of actual problem behind this change?

Thanks
Kevin

> 
> Signed-off-by: Neo Jia <cjia at nvidia.com>
> Signed-off-by: Kirti Wankhede <kwankhede at nvidia.com>
> ---
>  Documentation/ABI/testing/sysfs-bus-vfio-mdev | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/Documentation/ABI/testing/sysfs-bus-vfio-mdev
> b/Documentation/ABI/testing/sysfs-bus-vfio-mdev
> index 452dbe39270e..69e1291479ce 100644
> --- a/Documentation/ABI/testing/sysfs-bus-vfio-mdev
> +++ b/Documentation/ABI/testing/sysfs-bus-vfio-mdev
> @@ -85,6 +85,19 @@ Users:
>  		a particular <type-id> that can help in understanding the
>  		features provided by that type of mediated device.
> 
> +What:           /sys/.../mdev_supported_types/<type-
> id>/multiple_mdev_support
> +Date:           September 2018
> +Contact:        Kirti Wankhede <kwankhede at nvidia.com>
> +Description:
> +		Reading this attribute will return 0 or 1. Returning 1
> indicates
> +		multiple mdev devices of a particular <type-id> assigned to
> one
> +		User space application is supported by vendor driver. This is
> +		optional and readonly attribute.
> +Users:
> +		Userspace applications interested in knowing if multiple
> mdev
> +		devices of a particular <type-id> can be assigned to one
> +		instance of application.
> +
>  What:           /sys/.../<device>/<UUID>/
>  Date:           October 2016
>  Contact:        Kirti Wankhede <kwankhede at nvidia.com>
> --
> 2.7.0