Re: [PATCH] microcode: do not WARN_ON(cpu != 0) during resume

[Anton Arapov <anton@redhat.com>]

Andrew,

  may we have this patch in -mm? It has an ack from Jeremy as well.

thanks,
Anton.

On Thu, Dec 17, 2009 at 01:53:44PM +0000, 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.
> 
> 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@citrix.com>
> Cc: Jeremy Fitzhardinge <jeremy@goop.org>
> Cc: Dmitry Adamushko <dmitry.adamushko@gmail.com>
> Cc: Hugh Dickins <hugh@veritas.com>
> Cc: Ingo Molnar <mingo@elte.hu>
Acked-by: Anton Arapov <aarapov@redhat.com>

> ---
>  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);
>  
> -- 
> 1.5.6.5
> 
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> 

-- 
Anton