Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1163103AbdD0Qqq (ORCPT ); Thu, 27 Apr 2017 12:46:46 -0400 Received: from mail.kernel.org ([198.145.29.136]:32862 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S968010AbdD0Qqk (ORCPT ); Thu, 27 Apr 2017 12:46:40 -0400 MIME-Version: 1.0 In-Reply-To: <47640bc3-02e6-ceb1-44bd-2c21e64b4d37@oracle.com> References: <47640bc3-02e6-ceb1-44bd-2c21e64b4d37@oracle.com> From: Andy Lutomirski Date: Thu, 27 Apr 2017 09:46:09 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: xen_exit_mmap() questions To: Boris Ostrovsky Cc: Andy Lutomirski , "xen-devel@lists.xenproject.org" , Juergen Gross , X86 ML , Borislav Petkov , "linux-kernel@vger.kernel.org" Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2559 Lines: 67 On Thu, Apr 27, 2017 at 6:21 AM, Boris Ostrovsky wrote: > >>>>> >>>>>> Also, this code in drop_other_mm_ref() looks dubious to me: >>>>>> >>>>>> /* If this cpu still has a stale cr3 reference, then make sure >>>>>> it has been flushed. */ >>>>>> if (this_cpu_read(xen_current_cr3) == __pa(mm->pgd)) >>>>>> load_cr3(swapper_pg_dir); >>>>>> >>>>>> If cr3 hasn't been flushed to the hypervisor because we're in a lazy >>>>>> mode, why would load_cr3() help? Shouldn't this be xen_mc_flush() >>>>>> instead? >>>>> >>>>> load_cr3() actually ends with xen_mc_flush() by way of xen_write_cr3() >>>>> -> xen_mc_issue(). >>>> >>>> xen_mc_issue() does: >>>> >>>> if ((paravirt_get_lazy_mode() & mode) == 0) >>>> xen_mc_flush(); >>>> >>>> I assume the load_cr3() is intended to deal with the case where we're >>>> in lazy mode, but we'll still be in lazy mode, right? Or does it >>>> serve some other purpose? >>> >>> Of course. I can't read (I ignored the "== 0" part). >>> >>> Apparently the early version had an explicit flush but then it disappeared >>> (commit 9f79991d4186089e228274196413572cc000143b). >>> >>> The point of CR3 loading here, I believe, is to make sure the hypervisor >>> knows that the (v)CPU is no longer using the the mm's cr3 (we are loading >>> swapper_pgdir here). >> But that's what leave_mm() does. To be fair, the x86 lazy TLB >> management is a big mess, and this came up because I'm trying to clean >> it up without removing it. > > True. I don't know though if you can guarantee that leave_mm() (or > load_cr3() inside it) is actually called if we are in lazy mode. The code just before that makes these calls. Anyway, I propose to rewrite the whole thing like this: https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/tlbflush_cleanup&id=ff143a54bb3bafaaad6e32145a9cfbc112e8584f > >> >> I suppose I can try to keep xen_exit_mmap() working. Is there a >> simple way to try to unpin but to not treat it as an error if the >> hypervisor rejects it? > > Even if we managed to craft a call in Linux to do this (current > xen_pgd_unpin() will result in a WARNing in xen_mc_flush()) this will > still cause a bunch of warnings in the hypervisor (if it is built as > DEBUG, but bad nevertheless). > > But even without that, it is an error for a reason so how are you > planning to continue if you ignore it? > I was imagining that we'd just try to unpin and carry on if it fails. We can always unpin later in xen_pgd_free(). --Andy