linux-kernel - Re: [PATCH] microcode: do not WARN

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Date:	Thu, 17 Dec 2009 10:45:36 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Ian Campbell <ian.campbell@...rix.com>
CC:	linux-kernel@...r.kernel.org,
	Dmitry Adamushko <dmitry.adamushko@...il.com>,
	Hugh Dickins <hugh@...itas.com>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] microcode: do not WARN_ON(cpu != 0) during resume

On 12/17/2009 05:53 AM, Ian Campbell wrote:
> 871b72dd "x86: microcode: use smp_call_function_single instead of
> set_cpus_allowed, cleanup of synchronization logic" included:
>
>   static int mc_sysdev_resume(struct sys_device *dev)
>   {
> [...]
> +       /*
> +        * All non-bootup cpus are still disabled,
> +        * so only CPU 0 will apply ucode here.
> +        *
> +        * Moreover, there can be no concurrent
> +        * updates from any other places at this point.
> +        */
> +       WARN_ON(cpu != 0);
>
> However suspend/resume under Xen doesn't need to hot unplug all the CPUs, so we
> don't; the hypervisor can manage the context save/restore for all CPUs.
>    

Did you see a problem with this in practice, or just by inspection?

The Xen microcode driver will only load in a privileged domain, so I 
don't think this path can ever be exercised.

Regardless, the Xen microcode driver changes aren't upstream yet, so 
there's no need to apply this there yet.

Thanks,
     J

> It would be unnecessary to load microcode.ko in a Xen domU but if it does occur
> (e.g. because a distro installs the tools by default) we would like to avoid
> the warning on resume.
>
> Since the real constraint here is that we are running on the CPU for which we
> would like to load microcode (which in all practical circumstances is CPU0)
> just check for that and return if we are resuming a different CPU.
>
> There is no danger of concurrent updates, even if we ignore the fact that all
> but one CPUs are unplugged on native, because sysdev_resume() is single
> threaded.
>
> Signed-off-by: Ian Campbell<ian.campbell@...rix.com>
> Cc: Jeremy Fitzhardinge<jeremy@...p.org>
> Cc: Dmitry Adamushko<dmitry.adamushko@...il.com>
> Cc: Hugh Dickins<hugh@...itas.com>
> Cc: Ingo Molnar<mingo@...e.hu>
> ---
>   arch/x86/kernel/microcode_core.c |   11 +----------
>   1 files changed, 1 insertions(+), 10 deletions(-)
>
> diff --git a/arch/x86/kernel/microcode_core.c b/arch/x86/kernel/microcode_core.c
> index 378e9a8..1153062 100644
> --- a/arch/x86/kernel/microcode_core.c
> +++ b/arch/x86/kernel/microcode_core.c
> @@ -438,18 +438,9 @@ static int mc_sysdev_resume(struct sys_device *dev)
>   	int cpu = dev->id;
>   	struct ucode_cpu_info *uci = ucode_cpu_info + cpu;
>
> -	if (!cpu_online(cpu))
> +	if (cpu != smp_processor_id())
>   		return 0;
>
> -	/*
> -	 * All non-bootup cpus are still disabled,
> -	 * so only CPU 0 will apply ucode here.
> -	 *
> -	 * Moreover, there can be no concurrent
> -	 * updates from any other places at this point.
> -	 */
> -	WARN_ON(cpu != 0);
> -
>   	if (uci->valid&&  uci->mc)
>   		microcode_ops->apply_microcode(cpu);
>
>    

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