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

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Message-ID: <Pine.LNX.4.64.0703252334110.4535@chino.kir.corp.google.com>
Date:	Sun, 25 Mar 2007 23:35:53 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Zachary Amsden <zach@...are.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}

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?
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