From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:54207) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UZJtB-0006Ds-EH for qemu-devel@nongnu.org; Mon, 06 May 2013 07:47:30 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1UZJt9-0004FQ-ER for qemu-devel@nongnu.org; Mon, 06 May 2013 07:47:29 -0400 Received: from mx1.redhat.com ([209.132.183.28]:16403) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UZJt9-0004Eo-6R for qemu-devel@nongnu.org; Mon, 06 May 2013 07:47:27 -0400 Message-ID: <51879844.1030101@redhat.com> Date: Mon, 06 May 2013 13:47:16 +0200 From: Paolo Bonzini MIME-Version: 1.0 References: <1353808984-22368-1-git-send-email-qemulist@gmail.com> <51829B30.7020308@siemens.com> <51836FA8.2000501@siemens.com> <5184D942.5020506@redhat.com> <5184E601.9030407@web.de> <518764CD.2080602@redhat.com> <51876C86.5040301@siemens.com> <5187859C.5020305@redhat.com> <51878C4B.7050700@siemens.com> <51878CF0.1080009@redhat.com> <51878FCE.2020907@siemens.com> <518793F7.1010502@redhat.com> <51879674.70706@siemens.com> In-Reply-To: <51879674.70706@siemens.com> Content-Type: text/plain; charset=ISO-8859-15 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH v7 0/7] push mmio dispatch out of big lock List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Jan Kiszka Cc: qemu-devel , Liu Ping Fan Il 06/05/2013 13:39, Jan Kiszka ha scritto: > On 2013-05-06 13:28, Paolo Bonzini wrote: >> Il 06/05/2013 13:11, Jan Kiszka ha scritto: >>> On 2013-05-06 12:58, Paolo Bonzini wrote: >>>> Il 06/05/2013 12:56, Jan Kiszka ha scritto: >>>>>> The problem is that even if I/O for a region is supposed to happen >>>>>> within the BQL, lookup can happen outside the BQL. Lookup will use the >>>>>> region even if it is just to discard it: >>>>>> >>>>>> VCPU thread (under BQL) device thread >>>>>> -------------------------------------------------------------------------------------- >>>>>> flatview_ref >>>>>> memory_region_find returns d->mr >>>>>> memory_region_ref(d->mr) /* nop */ >>>>>> qdev_free(d) >>>>>> object_unparent(d) >>>>>> unrealize(d) >>>>>> memory_region_del_subregion(d->mr) >>>>>> FlatView updated, d->mr not in the new view >>>>>> >>>>>> flatview_unref >>>>>> memory_region_unref(d->mr) >>>>>> object_unref(d) >>>>>> free(d) >>>>>> if (!d->mr->is_ram) { /* BAD! */ >>>>>> memory_region_unref(d->mr) /* nop */ >>>>>> return error >>>>>> } >>>>>> >>>>>> >>>>>> Here, the memory region is dereferenced *before* we know that it is BQL-free >>>>>> (in fact, exactly to ascertain whether it is BQL-free). >>>>> >>>>> Both flatview update and lookup *plus* locking type evaluation (i.e. >>>>> memory region dereferencing) always happen under the address space lock. >>>>> See Pingfan's patch. >>>> >>>> That's true of address_space_rw/map, but I don't think it holds for >>>> memory_region_find. >>> >>> It has to, or it would be broken: Either it is called on a region that >>> supports reference counting >> >> You cannot know that in advance, can you? The address is decided by the >> guest. > > Need to help me again to get the context: In which case is this a > hot-path that we want to keep BQL-free? Current users of > memory_region_find appear to be all relatively slow paths, thus are fine > with staying under BQL. virtio-blk-dataplane is basically redoing memory_region_find with a separate data structure, exactly so that it can run outside the BQL before we get BQL-free MMIO dispatch. I can try to post patches later today that actually use memory_region_find instead. >>> and, thus, increments the counter before >>> returning, or it has to be called with the BQL held. >> >> ... or we need to support reference counting on all regions, so that the >> other possibility is automatically true. >> >> Strictly speaking, only regions that can be unplugged need to support >> reference counting. > > That should make the conversion, if actually required, more bearable. > Having to assign an owner to every region around as a precondition to > introduce a new concept with initially less than a handful of users > would be too much, I suppose. I agree. Though I have some hope that it could be scripted, this exception would make the conversion possible for non-qdevified devices. Paolo