Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754195AbbBTKr5 (ORCPT ); Fri, 20 Feb 2015 05:47:57 -0500 Received: from smtp.citrix.com ([66.165.176.89]:50727 "EHLO SMTP.CITRIX.COM" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752029AbbBTKrz convert rfc822-to-8bit (ORCPT ); Fri, 20 Feb 2015 05:47:55 -0500 X-IronPort-AV: E=Sophos;i="5.09,614,1418083200"; d="scan'208";a="228306158" Message-ID: <54E710D8.9020605@citrix.com> Date: Fri, 20 Feb 2015 10:47:52 +0000 From: Andrew Cooper User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Icedove/31.4.0 MIME-Version: 1.0 To: Linus Torvalds , "Kirill A. Shutemov" CC: "linux-kernel@vger.kernel.org" , "Xen-devel@lists.xen.org" , David Vrabel , Mel Gorman Subject: Re: [Xen-devel] NUMA_BALANCING and Xen PV guest regression in 3.20-rc0 References: <54E5DFED.9050700@citrix.com> <20150220010506.GA10837@node.dhcp.inet.fi> In-Reply-To: Content-Type: text/plain; charset="windows-1252" Content-Transfer-Encoding: 8BIT X-DLP: MIA2 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1992 Lines: 48 On 20/02/15 01:49, Linus Torvalds wrote: > On Thu, Feb 19, 2015 at 5:05 PM, Kirill A. Shutemov > wrote: >> I'm feeling I miss very basic background on how Xen works, but why does it >> set _PAGE_GLOBAL on userspace entries? It sounds strange to me. > It is definitely strange. I'm guessing that it's some ancient Xen hack > for the early Intel virtualization that used to have absolutely > horrendous vmenter/exit costs, including very much the TLB overhead. \ > > These days, Intel has address space identifiers, and doesn't flush the > whole TLB on VM entry/exit, so it's probably pointless to play games > with the global bit. It was introduced in 2006, but has nothing to do with VT-x http://xenbits.xen.org/gitweb/?p=xen.git;a=commitdiff;h=6f562e72cdc4b7e1519e23be75f812aebbf41db3 As long mode drops segment limit checking, the only way to protect a 64bit PV kernel from its userspace (both of which run in ring3 on user pages) is to maintain two sets of pagetables and switch between them on guest kernel/user context switches. The user set lack kernel mappings. I can't comment about the performance impact of the patch (way before my time), but the justification was to try and reduce the overhead of guest context switches. > > I get the feeling that a lot of Xen stuff is that kind of "legacy > hacks" that should just be cleaned up, but nobody has the energy or > the interest. Time, mainly. There certainly are areas which should be up for re-evaluation, given 9 years of change in hardware. > There was the whole odd crazy SHARED_KERNEL_PMD hackery too. SHARED_KERNEL_PMD is an artefact of Xen living in the same virtual address space as a PV guest, and needing to maintain linear mappings. ~Andrew -- 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/