Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933594AbbLOPYp (ORCPT ); Tue, 15 Dec 2015 10:24:45 -0500 Received: from prv-mh.provo.novell.com ([137.65.248.74]:37606 "EHLO prv-mh.provo.novell.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753884AbbLOPYo convert rfc822-to-8bit (ORCPT ); Tue, 15 Dec 2015 10:24:44 -0500 Message-Id: <56703EC802000078000BFB30@prv-mh.provo.novell.com> X-Mailer: Novell GroupWise Internet Agent 14.2.0 Date: Tue, 15 Dec 2015 08:24:40 -0700 From: "Jan Beulich" To: "Boris Ostrovsky" , "Konrad Rzeszutek Wilk" Cc: <#@char.us.oracle.com>, <3.14+@char.us.oracle.com>, , , , Subject: Re: [PATCH] xen/x86/pvh: Use HVM's flush_tlb_others op References: <1449966355-10611-1-git-send-email-boris.ostrovsky@oracle.com> <20151214152713.GC23203@char.us.oracle.com> <56702554.6000504@oracle.com> <567039EC02000078000BFAE3@prv-mh.provo.novell.com> <56702E69.4030503@oracle.com> In-Reply-To: <56702E69.4030503@oracle.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: 2383 Lines: 53 >>> On 15.12.15 at 16:14, wrote: > On 12/15/2015 10:03 AM, Jan Beulich wrote: >>>>> On 15.12.15 at 15:36, wrote: >>> On 12/14/2015 10:27 AM, Konrad Rzeszutek Wilk wrote: >>>> On Sat, Dec 12, 2015 at 07:25:55PM -0500, Boris Ostrovsky wrote: >>>>> Using MMUEXT_TLB_FLUSH_MULTI doesn't buy us much since the hypervisor >>>>> will likely perform same IPIs as would have the guest. >>>>> >>>> But if the VCPU is asleep, doing it via the hypervisor will save us waking >>>> up the guest VCPU, sending an IPI - just to do an TLB flush >>>> of that CPU. Which is pointless as the CPU hadn't been running the >>>> guest in the first place. >>>> >>>>> More importantly, using MMUEXT_INVLPG_MULTI may not to invalidate the >>>>> guest's address on remote CPU (when, for example, VCPU from another >>>>> guest >>>>> is running there). >>>> Right, so the hypervisor won't even send an IPI there. >>>> >>>> But if you do it via the normal guest IPI mechanism (which are opaque >>>> to the hypervisor) you and up scheduling the guest VCPU to do >>>> send an hypervisor callback. And the callback will go the IPI routine >>>> which will do an TLB flush. Not necessary. >>>> >>>> This is all in case of oversubscription of course. In the case where >>>> we are fine on vCPU resources it does not matter. >>> >>> So then should we keep these two operations (MMUEXT_INVLPG_MULTI and >>> MMUEXT_TLB_FLUSH_MULT) available to HVM/PVH guests? If the guest's VCPU >>> is not running then TLBs must have been flushed. >> While I followed the discussion, it didn't become clear to me what >> uses these are for HVM guests considering the separate address >> spaces. > > To avoid unnecessary IPIs to VCPUs that are not currently scheduled (my > mistake was that I didn't realize that IPIs to those pCPUs will be > filtered out by the hypervisor). > >> As long as they're useless if called, I'd still favor making >> them inaccessible. > > VCPUs that are scheduled will receive the required flush requests. I don't follow - an INVLPG done by the hypervisor won't do any flushing for a HVM guest. 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/