Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965068AbdD0A5b (ORCPT ); Wed, 26 Apr 2017 20:57:31 -0400 Received: from userp1040.oracle.com ([156.151.31.81]:46094 "EHLO userp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754764AbdD0A5W (ORCPT ); Wed, 26 Apr 2017 20:57:22 -0400 Subject: Re: xen_exit_mmap() questions To: Andy Lutomirski References: Cc: "xen-devel@lists.xenproject.org" , Juergen Gross , X86 ML , Borislav Petkov , "linux-kernel@vger.kernel.org" From: Boris Ostrovsky Message-ID: Date: Wed, 26 Apr 2017 20:55:55 -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; format=flowed Content-Transfer-Encoding: 7bit X-Source-IP: aserv0022.oracle.com [141.146.126.234] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2380 Lines: 65 On 04/26/2017 06:49 PM, Andy Lutomirski wrote: > On Wed, Apr 26, 2017 at 3:45 PM, Boris Ostrovsky > wrote: >> On 04/26/2017 04:52 PM, Andy Lutomirski wrote: >>> I was trying to understand xen_drop_mm_ref() to update it for some >>> changes I'm working on, and I'm wondering whether we need >>> xen_exit_mmap() at all. >>> >>> AFAICS the intent is to force all CPUs to drop their lazy uses of the >>> mm being destroyed so it can be unpinned before tearing down the page >>> tables, thus making it faster to tear down the page tables. This >>> seems like it'll speed up xen_set_pud() and xen_set_pmd(), but this >>> seems like it may be of rather limited value. >> >> Why do you think it's of limited value? Without it we will end up with a >> hypercall for each update. >> >> Or is your point that the number of those update is relatively small >> when we are tearing down? > > The latter. Also, unless I'm missing something, xen_set_pte() doesn't > have the optimization. I haven't looked at exactly how page table > teardown works, but if it clears each PTE individually, then that's > the bulk of the work. > >> >> >>> Could we get away with >>> deleting it? >>> >>> 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). -boris