From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759652AbdKPL5M (ORCPT ); Thu, 16 Nov 2017 06:57:12 -0500 Received: from mga07.intel.com ([134.134.136.100]:28208 "EHLO mga07.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759577AbdKPL5F (ORCPT ); Thu, 16 Nov 2017 06:57:05 -0500 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.44,402,1505804400"; d="scan'208";a="2238969" Message-ID: <5A0D7D9A.1060509@intel.com> Date: Thu, 16 Nov 2017 19:59:22 +0800 From: Wei Wang User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 MIME-Version: 1.0 To: "Michael S. Tsirkin" CC: virtio-dev@lists.oasis-open.org, linux-kernel@vger.kernel.org, qemu-devel@nongnu.org, virtualization@lists.linux-foundation.org, kvm@vger.kernel.org, linux-mm@kvack.org, mhocko@kernel.org, akpm@linux-foundation.org, mawilcox@microsoft.com, david@redhat.com, penguin-kernel@I-love.SAKURA.ne.jp, cornelia.huck@de.ibm.com, mgorman@techsingularity.net, aarcange@redhat.com, amit.shah@redhat.com, pbonzini@redhat.com, willy@infradead.org, liliang.opensource@gmail.com, yang.zhang.wz@gmail.com, quan.xu@aliyun.com, Nitesh Narayan Lal , Rik van Riel Subject: Re: [PATCH v17 6/6] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_VQ References: <1509696786-1597-1-git-send-email-wei.w.wang@intel.com> <1509696786-1597-7-git-send-email-wei.w.wang@intel.com> <5A097548.8000608@intel.com> <20171113192309-mutt-send-email-mst@kernel.org> <5A0ADB3B.4070407@intel.com> <20171114230805-mutt-send-email-mst@kernel.org> <5A0BB8EE.4050402@intel.com> <20171115152307-mutt-send-email-mst@kernel.org> In-Reply-To: <20171115152307-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/15/2017 09:26 PM, Michael S. Tsirkin wrote: > On Wed, Nov 15, 2017 at 11:47:58AM +0800, Wei Wang wrote: >> On 11/15/2017 05:21 AM, Michael S. Tsirkin wrote: >>> On Tue, Nov 14, 2017 at 08:02:03PM +0800, Wei Wang wrote: >>>> On 11/14/2017 01:32 AM, Michael S. Tsirkin wrote: >>>>>> - guest2host_cmd: written by the guest to ACK to the host about the >>>>>> commands that have been received. The host will clear the corresponding >>>>>> bits on the host2guest_cmd register. The guest also uses this register >>>>>> to send commands to the host (e.g. when finish free page reporting). >>>>> I am not sure what is the role of guest2host_cmd. Reporting of >>>>> the correct cmd id seems sufficient indication that guest >>>>> received the start command. Not getting any more seems sufficient >>>>> to detect stop. >>>>> >>>> I think the issue is when the host is waiting for the guest to report pages, >>>> it does not know whether the guest is going to report more or the report is >>>> done already. That's why we need a way to let the guest tell the host "the >>>> report is done, don't wait for more", then the host continues to the next >>>> step - sending the non-free pages to the destination. The following method >>>> is a conclusion of other comments, with some new thought. Please have a >>>> check if it is good. >>> config won't work well for this IMHO. >>> Writes to config register are hard to synchronize with the VQ. >>> For example, guest sends free pages, host says stop, meanwhile >>> guest sends stop for 1st set of pages. >> I still don't see an issue with this. Please see below: >> (before jumping into the discussion, just make sure I've well explained this >> point: now host-to-guest commands are done via config, and guest-to-host >> commands are done via the free page vq) > This is fine by me actually. But right now you have guest to host > not going through vq, going through command register instead - > this is how sending stop to host seems to happen. > If you make it go through vq then I think all will be well. > >> Case: Host starts to request the reporting with cmd_id=1. Some time later, >> Host writes "stop" to config, meantime guest happens to finish the reporting >> and plan to actively send a "stop" command from the free_page_vq(). >> Essentially, this is like a sync between two threads - if we view >> the config interrupt handler as one thread, another is the free page >> reporting worker thread. >> >> - what the config handler does is simply: >> 1.1: WRITE_ONCE(vb->reporting_stop, true); >> >> - what the reporting thread will do is >> 2.1: WRITE_ONCE(vb->reporting_stop, true); >> 2.2: send_stop_to_host_via_vq(); >> >> From the guest point of view, no matter 1.1 is executed first or 2.1 first, >> it doesn't make a difference to the end result - vb->reporting_stop is set. >> >> From the host point of view, it knows that cmd_id=1 has truly stopped the >> reporting when it receives a "stop" sign via the vq. >> >> >>> How about adding a buffer with "stop" in the VQ instead? >>> Wastes a VQ entry which you will need to reserve for this >>> but is it a big deal? >> The free page vq is guest-to-host direction. > Yes, for guest to host stop sign. > >> Using it for host-to-guest >> requests will make it bidirectional, which will result in the same issue >> described before: https://lkml.org/lkml/2017/10/11/1009 (the first response) >> >> On the other hand, I think adding another new vq for host-to-guest >> requesting doesn't make a difference in essence, compared to using config >> (same 1.1, 2.1, 2.2 above), but will be more complicated. > I agree with this. Host to guest can just incremenent the "free command id" > register. OK, thanks for the suggestions. I think one more issue left here: Previously, when the guest receives a config interrupt, it blindly adds the balloon work item to the workqueue in virtballoon_changed(), because only ballooning uses the config. Now, free page reporting is requested via config, too. We have the following two options: Option 1: add "diff = towards_target()" to virtballoon_changed(), and if diff = 0, it will not add the balloon work item to the wq. Option 2: add "cmd" for the host-to-guest request, and add the item when "cmd | CMD_BALLOON" is true. I'm inclined to take option 1 now. Which one would you prefer? Best, Wei