linux-kernel - Re: [PATCH 1/4] Pte drop ptep_get_and

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Message-ID: <4624CC7C.6040107@goop.org>
Date:	Tue, 17 Apr 2007 09:32:44 -0400
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Zachary Amsden <zach@...are.com>
CC:	Andrew Morton <akpm@...l.org>, Andi Kleen <ak@....de>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Chris Wright <chrisw@...s-sol.org>,
	Hugh Dickins <hugh@...itas.com>,
	David Rientjes <rientjes@...gle.com>,
	Michel Lespinasse <walken@...are.com>,
	Virtualization Mailing List <virtualization@...ts.osdl.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] Pte drop ptep_get_and_clear paravirt op.patch

Zachary Amsden wrote:
> In shadow mode hypervisors, ptep_get_and_clear achieves the desired
> purpose of keeping the shadows in sync by issuing a native_get_and_clear,
> followed by a call to pte_update, which indicates the PTE has been
> modified.
>
> Direct mode hypervisors (Xen) have no need for this anyway, and will trap
> the update using writable pagetables.
>   

Jan Beulich just posted a patch for Xen/linux which specialzes
ptep_get_and_clear, which apparently improves kernel-compile performance
by 25-30% on some configurations.  I think we'll want to keep this.

    J
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