linux-kernel - Re: [PATCH RFC] vm_unmap_aliases: allow callers to inhibit TLB flush

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Date:	Mon, 15 Dec 2008 17:28:19 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Nick Piggin <nickpiggin@...oo.com.au>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Arjan van de Ven <arjan@...ux.intel.com>
Subject: Re: [PATCH RFC] vm_unmap_aliases: allow callers to inhibit TLB flush

Nick Piggin wrote:
> On Friday 12 December 2008 12:59, Jeremy Fitzhardinge wrote:
>   
>> Nick Piggin wrote:
>>     
>>> Hi,
>>>
>>> On Friday 12 December 2008 06:05, Jeremy Fitzhardinge wrote:
>>>       
>>>> Hi Nick,
>>>>
>>>> In Xen when we're killing the lazy vmalloc aliases, we're only concerned
>>>> about the pagetable references to the mapped pages, not the TLB entries.
>>>>         
>>> Hm? Why is that? Why wouldn't it matter if some page table page gets
>>> written to via a stale TLB?
>>>       
>> No.  Well, yes, it would, but Xen itself will do whatever tlb flushes
>> are necessary to keep it safe (it must, since it doesn't trust guest
>> kernels).  It's fairly clever about working out which cpus need flushing
>> and if other flushes have already done the job.
>>     
>
> OK. Yeah, then the problem is simply that the guest may reuse that virtual
> memory for another vmap.
>   

Hm.  What you would you think of a "deferred tlb flush" flag (or 
something) to cause the next vmap to do the tlb flushes, in the case the 
vunmap happens in a context where the flushes can't be done?

    J
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