linux-kernel - Re: [PATCH v9 1/2] x86/kexec: VMCLEAR VMCSs loaded on all cpus if necessary

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Message-ID: <50BEFDF9.9080601@cn.fujitsu.com>
Date:	Wed, 05 Dec 2012 15:55:37 +0800
From:	Zhang Yanfei <zhangyanfei@...fujitsu.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	"x86@...nel.org" <x86@...nel.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Gleb Natapov <gleb@...hat.com>,
	"kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v9 1/2] x86/kexec: VMCLEAR VMCSs loaded on all cpus if
 necessary

于 2012年12月05日 04:14, Eric W. Biederman 写道:
> Zhang Yanfei <zhangyanfei@...fujitsu.com> writes:
> 
>> This patch provides a way to VMCLEAR VMCSs related to guests
>> on all cpus before executing the VMXOFF when doing kdump. This
>> is used to ensure the VMCSs in the vmcore updated and
>> non-corrupted.
> 
> Apologies for the delay I have been travelling, and I wanted
> to at least read through the code.
> 
> Overall I think this is good but I have one nit, and I see one real
> problem with this code.
> 
>> +/*
>> + * This is used to VMCLEAR all VMCSs loaded on the
>> + * processor. And when loading kvm_intel module, the
>> + * callback function pointer will be assigned.
>> + */
>> +void (*crash_vmclear_loaded_vmcss)(void) = NULL;
>> +EXPORT_SYMBOL_GPL(crash_vmclear_loaded_vmcss);
>> +
>> +static inline void cpu_emergency_vmclear_loaded_vmcss(void)
>> +{
>> +	if (crash_vmclear_loaded_vmcss)
>> +		crash_vmclear_loaded_vmcss();
>> +}
> 
> The nit is the use of emergency instead of crash in the name.

ok, emergency -> crash

> 
> The problem is that this is potentially a NULL pointer dereference if
> kvm-intel is removed.  The easist fix would be in your second patch to
> just make it impossible to unload the kvm-intel module.  Otherwise
> there the deference of crash_vmclear_loaded_vmcss needs to be rcu
> protected, with a syncrhonize_rcu after the pointer is set to NULL in
> the unload path.

Ah, thanks for this comment.

I think I will use the rcu machanism to solve the problem.

> 
> Otherwise I have no objections to this code.

Thanks for your review. I will update the patch and resend it.

Thanks
Zhang Yanfei
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