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[209.132.183.28]) by mx.google.com with ESMTPS id s124si18009695qkb.5.2019.08.04.21.36.48 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sun, 04 Aug 2019 21:36:48 -0700 (PDT) Received-SPF: pass (google.com: domain of jasowang@redhat.com designates 209.132.183.28 as permitted sender) client-ip=209.132.183.28; Authentication-Results: mx.google.com; spf=pass (google.com: domain of jasowang@redhat.com designates 209.132.183.28 as permitted sender) smtp.mailfrom=jasowang@redhat.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 9FDEA30A5408; Mon, 5 Aug 2019 04:36:47 +0000 (UTC) Received: from [10.72.12.115] (ovpn-12-115.pek2.redhat.com [10.72.12.115]) by smtp.corp.redhat.com (Postfix) with ESMTP id B7AA560856; Mon, 5 Aug 2019 04:36:42 +0000 (UTC) Subject: Re: [PATCH V2 7/9] vhost: do not use RCU to synchronize MMU notifier with worker To: "Michael S. Tsirkin" , Jason Gunthorpe Cc: kvm@vger.kernel.org, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20190731084655.7024-1-jasowang@redhat.com> <20190731084655.7024-8-jasowang@redhat.com> <20190731123935.GC3946@ziepe.ca> <7555c949-ae6f-f105-6e1d-df21ddae9e4e@redhat.com> <20190731193057.GG3946@ziepe.ca> <20190801141512.GB23899@ziepe.ca> <42ead87b-1749-4c73-cbe4-29dbeb945041@redhat.com> <20190802124613.GA11245@ziepe.ca> <20190802100414-mutt-send-email-mst@kernel.org> From: Jason Wang Message-ID: Date: Mon, 5 Aug 2019 12:36:40 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <20190802100414-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.42]); Mon, 05 Aug 2019 04:36:47 +0000 (UTC) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 2019/8/2 下午10:27, Michael S. Tsirkin wrote: > On Fri, Aug 02, 2019 at 09:46:13AM -0300, Jason Gunthorpe wrote: >> On Fri, Aug 02, 2019 at 05:40:07PM +0800, Jason Wang wrote: >>>> This must be a proper barrier, like a spinlock, mutex, or >>>> synchronize_rcu. >>> >>> I start with synchronize_rcu() but both you and Michael raise some >>> concern. >> I've also idly wondered if calling synchronize_rcu() under the various >> mm locks is a deadlock situation. >> >>> Then I try spinlock and mutex: >>> >>> 1) spinlock: add lots of overhead on datapath, this leads 0 performance >>> improvement. >> I think the topic here is correctness not performance improvement > The topic is whether we should revert > commit 7f466032dc9 ("vhost: access vq metadata through kernel virtual address") > > or keep it in. The only reason to keep it is performance. Maybe it's time to introduce the config option? > > Now as long as all this code is disabled anyway, we can experiment a > bit. > > I personally feel we would be best served by having two code paths: > > - Access to VM memory directly mapped into kernel > - Access to userspace > > > Having it all cleanly split will allow a bunch of optimizations, for > example for years now we planned to be able to process an incoming short > packet directly on softirq path, or an outgoing on directly within > eventfd. It's not hard consider we've already had our own accssors. But the question is (as asked in another thread), do you want permanent GUP or still use MMU notifiers. Thanks