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Date:   Wed, 6 Jan 2021 11:08:56 +0000
From:   Wei Liu <wei.liu@kernel.org>
To:     Dexuan Cui <decui@microsoft.com>
Cc:     wei.liu@kernel.org, kys@microsoft.com, haiyangz@microsoft.com,
        sthemmin@microsoft.com, linux-hyperv@vger.kernel.org,
        linux-kernel@vger.kernel.org, mikelley@microsoft.com,
        vkuznets@redhat.com, marcelo.cerri@canonical.com
Subject: Re: [PATCH v2] Drivers: hv: vmbus: Add
 /sys/bus/vmbus/supported_features
Message-ID: <20210106110856.i23e7alpf2biirsu@liuwe-devbox-debian-v2>
References: <20210106000434.5551-1-decui@microsoft.com>
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On Tue, Jan 05, 2021 at 04:04:34PM -0800, Dexuan Cui wrote:
> 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.
> 
> Signed-off-by: Dexuan Cui <decui@microsoft.com>

LGTM. I will wait a bit for others to comment.

> ---
> 
> Change in v2:
> 	Added a "Values:" section
> 	Updated "Date:"
> 
>  Documentation/ABI/stable/sysfs-bus-vmbus |  9 +++++++++
>  drivers/hv/vmbus_drv.c                   | 20 ++++++++++++++++++++
>  2 files changed, 29 insertions(+)
> 
> diff --git a/Documentation/ABI/stable/sysfs-bus-vmbus b/Documentation/ABI/stable/sysfs-bus-vmbus
> index c27b7b89477c..c8d56389b7be 100644
> --- a/Documentation/ABI/stable/sysfs-bus-vmbus
> +++ b/Documentation/ABI/stable/sysfs-bus-vmbus
> @@ -1,3 +1,12 @@
> +What:		/sys/bus/vmbus/supported_features
> +Date:		Jan 2021
> +KernelVersion:	5.11
> +Contact:	Dexuan Cui <decui@microsoft.com>
> +Description:	Features specific to VMs running on Hyper-V
> +Values:		A list of strings.
> +		hibernation: the host toolstack supports hibernation for the VM.
> +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
>