Message-ID: <1460661066.4560.19.camel@decadent.org.uk>
Subject: Re: [PATCH 3.16.y-ckt 17/17] Drivers: hv: vmbus: prevent cpu
 offlining on newer hypervisors
From: Ben Hutchings <ben@decadent.org.uk>
To: "K. Y. Srinivasan" <kys@microsoft.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Chas Williams <3chas3@gmail.com>,
        Luis Henriques <luis.henriques@canonical.com>,
        linux-kernel@vger.kernel.org, stable@vger.kernel.org,
        kernel-team@lists.ubuntu.com
Date: Thu, 14 Apr 2016 20:11:06 +0100
In-Reply-To: <1460484484-22395-18-git-send-email-luis.henriques@canonical.com>
References: <1460484484-22395-1-git-send-email-luis.henriques@canonical.com>
	 <1460484484-22395-18-git-send-email-luis.henriques@canonical.com>
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On Tue, 2016-04-12 at 19:08 +0100, Luis Henriques wrote:
> 3.16.7-ckt27 -stable review patch.=C2=A0=C2=A0If anyone has any objection=
s, please let me know.
>=20
> ---8<------------------------------------------------------------
>=20
> From: Vitaly Kuznetsov <vkuznets@redhat.com>
>=20
> commit e513229b4c386e6c9f66298c13fde92f73e6e1ac upstream.
>=20
> When an SMP Hyper-V guest is running on top of 2012R2 Server and secondar=
y
> cpus are sent offline (with echo 0 > /sys/devices/system/cpu/cpu$cpu/onli=
ne)
> the system freeze is observed. This happens due to the fact that on newer
> hypervisors (Win8, WS2012R2, ...) vmbus channel handlers are distributed
> across all cpus (see init_vp_index() function in drivers/hv/channel_mgmt.=
c)
> and on cpu offlining nobody reassigns them to CPU0. Prevent cpu offlining
> when vmbus is loaded until the issue is fixed host-side.
>=20
> This patch also disables hibernation but it is OK as it is also broken (M=
CE
> error is hit on resume). Suspend still works.
[...]
> +static void hv_cpu_hotplug_quirk(bool vmbus_loaded)
> +{
> +	static void *previous_cpu_disable;
> +
> +	/*
> +	=C2=A0* Offlining a CPU when running on newer hypervisors (WS2012R2, Wi=
n8,
> +	=C2=A0* ...) is not supported at this moment as channel interrupts are
> +	=C2=A0* distributed across all of them.
> +	=C2=A0*/
> +
> +	if ((vmbus_proto_version =3D=3D VERSION_WS2008) ||
> +	=C2=A0=C2=A0=C2=A0=C2=A0(vmbus_proto_version =3D=3D VERSION_WIN7))
> +		return;
> +
> +	if (vmbus_loaded) {
> +		previous_cpu_disable =3D smp_ops.cpu_disable;
> +		smp_ops.cpu_disable =3D hyperv_cpu_disable;
> +		pr_notice("CPU offlining is not supported by hypervisor\n");
> +	} else if (previous_cpu_disable)
> +		smp_ops.cpu_disable =3D previous_cpu_disable;
[...]

This is a really bad hack. =C2=A0What if two different drivers patched
smp_ops and got unloaded in a different order? =C2=A0Perhaps the core
support code for Hyper-V should define its own smp_ops.

I don't want to stop this going into stable, but seriously, please
clean this up.

Ben.

--=20
Ben Hutchings
In a hierarchy, every employee tends to rise to his level of incompetence.
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