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[Xen-devel] Re: [PATCH 1 of 4] mm: add a ptep_modify_prot transaction ab

To: benh@xxxxxxxxxxxxxxxxxxx
Subject: [Xen-devel] Re: [PATCH 1 of 4] mm: add a ptep_modify_prot transaction abstraction
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Wed, 18 Jun 2008 16:59:05 -0700
Cc: Zachary Amsden <zach@xxxxxxxxxx>, Rusty Russell <rusty@xxxxxxxxxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>, kvm-devel <kvm-devel@xxxxxxxxxxxxxxxxxxxxx>, x86@xxxxxxxxxx, LKML <linux-kernel@xxxxxxxxxxxxxxx>, Virtualization Mailing List <virtualization@xxxxxxxxxxxxxx>, Hugh Dickins <hugh@xxxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxx>, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx>
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Benjamin Herrenschmidt wrote:
On Mon, 2008-06-16 at 04:30 -0700, Jeremy Fitzhardinge wrote:
The only current user of this interface is mprotect

Do you plan to use it with fork ultimately ?

Good point, I'd overlooked that. I guess that means using it in ptep_set_wrprotect().

At present the x86 ptep_set_wrprotect() just uses clear_bit on the pte, which is a locked cycle. Is that significantly cheaper than an xchg + set? (Same number of locked operations...)

   J

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