From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755372AbaFBQpn (ORCPT ); Mon, 2 Jun 2014 12:45:43 -0400 Received: from mail-wg0-f47.google.com ([74.125.82.47]:40788 "EHLO mail-wg0-f47.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754942AbaFBQpl (ORCPT ); Mon, 2 Jun 2014 12:45:41 -0400 Date: Mon, 2 Jun 2014 19:45:35 +0300 From: Gleb Natapov To: Andi Kleen Cc: Andi Kleen , peterz@infradead.org, pbonzini@redhat.com, eranian@google.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, mtosatti@redhat.com Subject: Re: [PATCH 4/4] kvm: Implement PEBS virtualization Message-ID: <20140602164534.GD4715@minantech.com> References: <1401412327-14810-1-git-send-email-andi@firstfloor.org> <1401412327-14810-5-git-send-email-andi@firstfloor.org> <20140530082136.GA4715@minantech.com> <20140530162424.GC8876@tassilo.jf.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140530162424.GC8876@tassilo.jf.intel.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, May 30, 2014 at 09:24:24AM -0700, Andi Kleen wrote: > > > To avoid any problems with guest pages being swapped by the host we > > > pin the pages when the PEBS buffer is setup, by intercepting > > > that MSR. > > It will avoid guest page to be swapped, but shadow paging code may still drop > > shadow PT pages that build a mapping from DS virtual address to the guest page. > > You're saying the EPT code could tear down the EPT mappings? Under memory pressure yes. mmu_shrink_scan() calls prepare_zap_oldest_mmu_page() which destroys oldest mmu pages like its name says. As far as I can tell running nested guest can also result in EPT mapping to be dropped since it will create a lot of shadow pages and this will cause make_mmu_pages_available() to destroy some shadow pages and it may choose EPT pages to destroy. CCing Marcelo to confirm/correct. > > OK that would need to be prevented too. Any suggestions how? Only high level. Mark shadow pages involved in translation we want to keep and skip them in prepare_zap_oldest_mmu_page(). > > > With EPT it is less likely to happen (but still possible IIRC depending on memory > > pressure and how much memory shadow paging code is allowed to use), without EPT > > it will happen for sure. > > Don't care about the non EPT case, this is white listed only for EPT supporting > CPUs. User may still disable EPT during module load, so pebs should be dropped from a guest's cpuid in this case. > > > There is nothing, as far as I can see, that says what will happen if the > > condition is not met. I always interpreted it as undefined behaviour so > > anything can happen including CPU dies completely. You are saying above > > on one hand that CPU cannot handle any kinds of faults during write to > > DS area, but on the other hand a guest could only crash itself. Is this > > architecturally guarantied? > > You essentially would get random page faults, and the PEBS event will > be cancelled. No hangs. Is this a guest who will get those random page faults or a host? > > It's not architecturally guaranteed, but we white list anyways so > we only care about the white listed CPUs at this point. For them > I have confirmation that it works. > > -Andi -- Gleb.