RE: [PATCH] Drivers: hv: vmbus: Add /sys/bus/vmbus/supported_features

[Michael Kelley <mikelley@microsoft.com>]

From: Dexuan Cui <decui@microsoft.com> Sent: Tuesday, December 22, 2020 4:12 PM
> 
> When a Linux VM runs on Hyper-V, if the host toolstack doesn't support
> hibernation for the VM (this happens on old Hyper-V hosts like Windows
> Server 2016, or new Hyper-V hosts if the admin or user doesn't declare
> the hibernation intent for the VM), the VM is discouraged from trying
> hibernation (because the host doesn't guarantee that the VM's virtual
> hardware configuration will remain exactly the same across hibernation),
> i.e. the VM should not try to set up the swap partition/file for
> hibernation, etc.
> 
> x86 Hyper-V uses the presence of the virtual ACPI S4 state as the
> indication of the host toolstack support for a VM. Currently there is
> no easy and reliable way for the userspace to detect the presence of
> the state (see https://lkml.org/lkml/2020/12/11/1097).  Add
> /sys/bus/vmbus/supported_features for this purpose.

I'm OK with surfacing the hibernation capability via an entry in
/sys/bus/vmbus.  Correct me if I'm wrong, but I think the concept
being surfaced is not "ACPI S4 state" precisely, but slightly more
generally whether hibernation is supported for the VM.  While
those two concepts may be 1:1 for the moment, there might be
future configurations where "hibernation is supported" depends
on other factors as well.

The guidance for things in /sys is that they generally should
be single valued (see Documentation/filesystems/sysfs.rst).  So my
recommendation is to create a "hibernation" entry that has a value
of 0 or 1.  That's the pattern I see in lots of other places in /sys.  If
other Hyper-V or VMbus-related features need to be surfaced in
the future, they would have their own single-valued entry.

Michael

> 
> Signed-off-by: Dexuan Cui <decui@microsoft.com>
> ---
>  Documentation/ABI/stable/sysfs-bus-vmbus |  7 +++++++
>  drivers/hv/vmbus_drv.c                   | 20 ++++++++++++++++++++
>  2 files changed, 27 insertions(+)
> 
> diff --git a/Documentation/ABI/stable/sysfs-bus-vmbus
> b/Documentation/ABI/stable/sysfs-bus-vmbus
> index c27b7b89477c..3ba765ae6695 100644
> --- a/Documentation/ABI/stable/sysfs-bus-vmbus
> +++ b/Documentation/ABI/stable/sysfs-bus-vmbus
> @@ -1,3 +1,10 @@
> +What:		/sys/bus/vmbus/supported_features
> +Date:		Dec 2020
> +KernelVersion:	5.11
> +Contact:	Dexuan Cui <decui@microsoft.com>
> +Description:	Features specific to VMs running on Hyper-V
> +Users:		Daemon that sets up swap partition/file for hibernation
> +
>  What:		/sys/bus/vmbus/devices/<UUID>/id
>  Date:		Jul 2009
>  KernelVersion:	2.6.31
> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> index d491fdcee61f..958487a40a18 100644
> --- a/drivers/hv/vmbus_drv.c
> +++ b/drivers/hv/vmbus_drv.c
> @@ -678,6 +678,25 @@ static const struct attribute_group vmbus_dev_group = {
>  };
>  __ATTRIBUTE_GROUPS(vmbus_dev);
> 
> +/* Set up bus attribute(s) for /sys/bus/vmbus/supported_features */
> +static ssize_t supported_features_show(struct bus_type *bus, char *buf)
> +{
> +	bool hb = hv_is_hibernation_supported();
> +
> +	return sprintf(buf, "%s\n", hb ? "hibernation" : "");
> +}
> +
> +static BUS_ATTR_RO(supported_features);
> +
> +static struct attribute *vmbus_bus_attrs[] = {
> +	&bus_attr_supported_features.attr,
> +	NULL,
> +};
> +static const struct attribute_group vmbus_bus_group = {
> +	.attrs = vmbus_bus_attrs,
> +};
> +__ATTRIBUTE_GROUPS(vmbus_bus);
> +
>  /*
>   * vmbus_uevent - add uevent for our device
>   *
> @@ -1024,6 +1043,7 @@ static struct bus_type  hv_bus = {
>  	.uevent =		vmbus_uevent,
>  	.dev_groups =		vmbus_dev_groups,
>  	.drv_groups =		vmbus_drv_groups,
> +	.bus_groups =		vmbus_bus_groups,
>  	.pm =			&vmbus_pm,
>  };
> 
> --
> 2.19.1