Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753169AbdHJT1p (ORCPT ); Thu, 10 Aug 2017 15:27:45 -0400 Received: from bombadil.infradead.org ([65.50.211.133]:35123 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752792AbdHJT1o (ORCPT ); Thu, 10 Aug 2017 15:27:44 -0400 Date: Thu, 10 Aug 2017 21:27:42 +0200 From: Peter Zijlstra To: Jork Loeser Cc: KY Srinivasan , Simon Xiao , Haiyang Zhang , Stephen Hemminger , "torvalds@linux-foundation.org" , "luto@kernel.org" , "hpa@zytor.com" , "vkuznets@redhat.com" , "linux-kernel@vger.kernel.org" , "rostedt@goodmis.org" , "andy.shevchenko@gmail.com" , "tglx@linutronix.de" , "mingo@kernel.org" , "linux-tip-commits@vger.kernel.org" Subject: Re: [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB flush Message-ID: <20170810192742.GJ6524@worktop.programming.kicks-ass.net> References: <20170802160921.21791-8-vkuznets@redhat.com> <20170810185646.GI6524@worktop.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.22.1 (2013-10-16) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1316 Lines: 31 On Thu, Aug 10, 2017 at 07:08:22PM +0000, Jork Loeser wrote: > > > Subject: Re: [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB flush > > > > Hold on.. if we don't IPI for TLB invalidation. What serializes our > > > software page table walkers like fast_gup() ? > > > > Hypervisor may implement this functionality via an IPI. > > > > K. Y > > HvFlushVirtualAddressList() states: > This call guarantees that by the time control returns back to the > caller, the observable effects of all flushes on the specified virtual > processors have occurred. > > HvFlushVirtualAddressListEx() refers to HvFlushVirtualAddressList() as adding sparse target VP lists. > > Is this enough of a guarantee, or do you see other races? That's nowhere near enough. We need the remote CPU to have completed any guest IF section that was in progress at the time of the call. So if a host IPI can interrupt a guest while the guest has IF cleared, and we then process the host IPI -- clear the TLBs -- before resuming the guest, which still has IF cleared, we've got a problem. Because at that point, our software page-table walker, that relies on IF being clear to guarantee the page-tables exist, because it holds off the TLB invalidate and thereby the freeing of the pages, gets its pages ripped out from under it.