Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S939660AbdD0UAz (ORCPT ); Thu, 27 Apr 2017 16:00:55 -0400 Received: from userp1040.oracle.com ([156.151.31.81]:38782 "EHLO userp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753780AbdD0UAr (ORCPT ); Thu, 27 Apr 2017 16:00:47 -0400 Subject: Re: xen_exit_mmap() questions To: Andy Lutomirski References: <47640bc3-02e6-ceb1-44bd-2c21e64b4d37@oracle.com> Cc: "xen-devel@lists.xenproject.org" , Juergen Gross , X86 ML , Borislav Petkov , "linux-kernel@vger.kernel.org" From: Boris Ostrovsky Message-ID: <89ea23ba-2fec-41e6-2628-c3a2fd789b1f@oracle.com> Date: Thu, 27 Apr 2017 16:00:24 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-Source-IP: userv0021.oracle.com [156.151.31.71] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2271 Lines: 52 On 04/27/2017 12:46 PM, Andy Lutomirski wrote: > 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. Yes, and I was unsure whether we always get to make these calls, based on mm and cpu_tlbstate. I think we do and with your changes it is made even more clear. > > 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 Can you explain xen_pgd_free() change? When do you expect xen_exit_mmap() to fail unpinning (compared to what we have now)? -boris