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

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Date:	Fri, 18 Dec 2009 08:43:04 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Ian Campbell <Ian.Campbell@...rix.com>
CC:	"linux-kernel@...r.kernel.org" <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 12/18/2009 01:07 AM, Ian Campbell wrote:
> On Thu, 2009-12-17 at 18:45 +0000, Jeremy Fitzhardinge wrote:
>    
>> 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?
>>      
> In practice.
>
>    
>> The Xen microcode driver will only load in a privileged domain, so I
>> don't think this path can ever be exercised.
>>      
> This was on bare v2.6.32 + your bugfixes branch, there was no domain 0
> support or Xen microcode driver.
>
> The regular native microcode driver can load in a domU and generally
> silently does the right thing (i.e. nothing) apart from this issue.
> Distro initscripts will tend to try and load the native driver and they
> are very unlikely to special case Xen domU to avoid it.
>    

OK, I see.  The patch looks reasonable to me then.

Alternatively we could try to prevent the native microcode drivers from 
loading in the Xen case, but since the code just checks the CPU vendor 
it doesn't give us much to work with aside from adding an explicit Xen 
test.  Making the resume code more robust seems like the better option.

     J

>    
>> Regardless, the Xen microcode driver changes aren't upstream yet, so
>> there's no need to apply this there yet.
>>      
> This issue is in the generic microcode code so it is also needed now. I
> guess once the Xen microcode driver is upstreamed it will fix domU by
> suppressing the driver from loading at all except in dom0.
>
> Ian.
>
>    
>> 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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