linux-kernel - Re: [PATCH] KVM: x86: Convert tsc_write_lock to raw

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Message-ID: <4D5024F3.9090105@siemens.com>
Date:	Mon, 07 Feb 2011 17:59:31 +0100
From:	Jan Kiszka <jan.kiszka@...mens.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Zachary Amsden <zamsden@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	kvm <kvm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86: Convert tsc_write_lock to raw_spinlock

On 2011-02-07 17:26, Avi Kivity wrote:
> On 02/07/2011 05:58 PM, Jan Kiszka wrote:
>> On 2011-02-07 16:52, Avi Kivity wrote:
>>>  On 02/07/2011 05:38 PM, Jan Kiszka wrote:
>>>>>
>>>>>   I don't know as it is allowed to sleep, it doesn't call any sleeping
>>>>>   functions to my knowledge.  What worries me in the RT case is that the
>>>>>   spinlock acquired for hardware_enable might be preempted and run on
>>>>>   another CPU, which obviously isn't what you want.
>>>>
>>>>  I see now, there are calls to raw_smp_processor_id.
>>>>
>>>>  I think it's best to make this a raw lock. At this chance, some
>>>>  read-only users of vm_list should be rcu'ified. Will have a look.
>>>
>>>  vm_list is rarely used, for either read or write.  I don't see the need
>>>  to rcu it.
>>
>> Avoid that code under this lock expands the preempt-disabled period,
>> specifically under -rt, and specifically as the number of objects over
>> which we loop is user-defined.
> 
> Good point; even under non-rt.
> 
> (well, actually, cpufreq_notifier and kvm_arch_hardware_enable are 
> already non preemptible, and the stats code should just go away?)

The stats code is trivial to convert, so it doesn't matter.

But what about mmu_shrink and its list_move_tail? How is this
synchronized against kvm_destroy_vm - already today?

Jan

-- 
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux
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