Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754149AbYJOPXw (ORCPT ); Wed, 15 Oct 2008 11:23:52 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751932AbYJOPXn (ORCPT ); Wed, 15 Oct 2008 11:23:43 -0400 Received: from vpn.id2.novell.com ([195.33.99.129]:46895 "EHLO vpn.id2.novell.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751760AbYJOPXm convert rfc822-to-8bit (ORCPT ); Wed, 15 Oct 2008 11:23:42 -0400 Message-Id: <48F6274D.76E4.0078.0@novell.com> X-Mailer: Novell GroupWise Internet Agent 8.0.0 Beta Date: Wed, 15 Oct 2008 16:24:29 +0100 From: "Jan Beulich" To: "Jeremy Fitzhardinge" , "Chris Lalancette" Cc: "xen-devel@lists.xensource.com" , Subject: Re: [Xen-devel] [PATCH]: Fix Xen domU boot with batched mprotect References: <48F5CE10.3060403@redhat.com> In-Reply-To: <48F5CE10.3060403@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 8BIT Content-Disposition: inline Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1455 Lines: 30 >>> Chris Lalancette 15.10.08 13:03 >>> >The right thing to do is to use arbitrary_virt_to_machine, so that we can be >sure we are modifying the right pfn. This unfortunately introduces a >performance penalty because of a full page-table-walk, but we can avoid that >penalty for pages in the p2m list by checking if virt_addr_valid is true, and if >so, just doing the lookup in the p2m table. Could you explain how virt_addr_valid() can validly be used here? Looking at its implementation #define virt_addr_valid(kaddr) pfn_valid(__pa(kaddr) >> PAGE_SHIFT) a kaddr in kmap space (i.e. above high_memory) would return a bogus physical address, and hence pfn_valid() on the resulting pfn is meaningless. I'd instead simply compare the address in question against high_memory, and perhaps instead of in arbitrary_virt_to_machine() in ptep_modify_prot_commit() under an #ifdef CONFIG_HIGHPTE. But performance-wise, CONFIG_HIGHPTE sucks under Xen anyway, so you'd better not turn this on in the first place. We may want/need to provide a means to disable this at run time so the same kernel when run natively could still make use of it, but without impacting performance under Xen. Jan -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/