linux-kernel - Re: [patch -mm 1/2] i386: add ptep_test_and_clear

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Message-ID: <46082B6B.8010707@vmware.com>
Date:	Mon, 26 Mar 2007 12:22:03 -0800
From:	Zachary Amsden <zach@...are.com>
To:	David Rientjes <rientjes@...gle.com>
CC:	Hugh Dickins <hugh@...itas.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [patch -mm 1/2] i386: add ptep_test_and_clear_{dirty,young}

David Rientjes wrote:
> On Sun, 25 Mar 2007, Zachary Amsden wrote:
>
>   
>> If you actually clear the bit, you need to:
>>
>> +         pte_update_defer(vma->vm_mm, addr, ptep);
>>
>> The reason is, when updating PTEs, the hypervisor must be notified.  Using
>> atomic operations to do this is fine for all hypervisors I am aware of.
>> However, for hypervisors which shadow page tables, if these PTE modifications
>> are not trapped, you need a post-modification call to fulfill the update of
>> the shadow page table.
>>
>>     
>
> Then why was ptep_test_and_clear_{dirty,young} ever removed in the first 
> place??  To gain the optimization of one fewer branch and introduce a hack 
> to advertise it's existance so the generic header file doesn't include its 
> own version?
>   

Yes, pretty much.

Zach
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