From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:49836) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1dq0iX-00089Y-1S for qemu-devel@nongnu.org; Thu, 07 Sep 2017 13:36:03 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1dq0iJ-0007GR-E9 for qemu-devel@nongnu.org; Thu, 07 Sep 2017 13:35:53 -0400 Received: from mail-wr0-x242.google.com ([2a00:1450:400c:c0c::242]:35840) by eggs.gnu.org with esmtps (TLS1.0:RSA_AES_128_CBC_SHA1:16) (Exim 4.71) (envelope-from ) id 1dq0iJ-0007Fo-50 for qemu-devel@nongnu.org; Thu, 07 Sep 2017 13:35:39 -0400 Received: by mail-wr0-x242.google.com with SMTP id g50so172817wra.3 for ; Thu, 07 Sep 2017 10:35:39 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <20170907171435.GE2194@work-vm> References: <1503471071-2233-1-git-send-email-peterx@redhat.com> <20170906145043.GG15535@stefanha-x1.localdomain> <20170906151436.GF2215@work-vm> <20170907093546.GE2098@work-vm> <20170907120227.GE23040@pxdev.xzpeter.org> <20170907171435.GE2194@work-vm> From: Stefan Hajnoczi Date: Thu, 7 Sep 2017 18:35:36 +0100 Message-ID: Content-Type: text/plain; charset="UTF-8" Subject: Re: [Qemu-devel] [RFC v2 0/8] monitor: allow per-monitor thread List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Dr. David Alan Gilbert" Cc: Peter Xu , Laurent Vivier , Fam Zheng , Michael Roth , Juan Quintela , qemu-devel , Markus Armbruster , Paolo Bonzini On Thu, Sep 7, 2017 at 6:14 PM, Dr. David Alan Gilbert wrote: > * Stefan Hajnoczi (stefanha@gmail.com) wrote: >> On Thu, Sep 7, 2017 at 1:02 PM, Peter Xu wrote: >> > On Thu, Sep 07, 2017 at 11:09:29AM +0100, Stefan Hajnoczi wrote: >> >> On Thu, Sep 7, 2017 at 10:35 AM, Dr. David Alan Gilbert >> >> wrote: >> >> > * Stefan Hajnoczi (stefanha@gmail.com) wrote: >> >> >> On Wed, Sep 6, 2017 at 4:14 PM, Dr. David Alan Gilbert >> >> >> wrote: >> >> >> > * Stefan Hajnoczi (stefanha@gmail.com) wrote: >> >> >> >> On Wed, Aug 23, 2017 at 02:51:03PM +0800, Peter Xu wrote: >> >> >> >> > The root problem is that, monitor commands are all handled in main >> >> >> >> > loop thread now, no matter how many monitors we specify. And, if main >> >> >> >> > loop thread hangs due to some reason, all monitors will be stuck. >> >> >> >> >> >> >> >> I see a larger issue with postcopy: existing QEMU code assumes that >> >> >> >> guest memory access is instantaneous. >> >> >> >> >> >> >> >> Postcopy breaks this assumption and introduces blocking points that can >> >> >> >> now take unbounded time. >> >> >> >> >> >> >> >> This problem isn't specific to the monitor. It can also happen to other >> >> >> >> components in QEMU like the gdbstub. >> >> >> >> >> >> >> >> Do we need an asynchronous memory API? Synchronous memory access should >> >> >> >> only be allowed in vcpu threads. >> >> >> > >> >> >> > It would probably be useful for gdbstub where the overhead of async >> >> >> > doesn't matter; but doing that for all IO emulation is hard. >> >> >> >> >> >> Why is it hard? >> >> >> >> >> >> Memory access can be synchronous in the vcpu thread. That eliminates >> >> >> a lot of code straight away. >> >> >> >> >> >> Anything using dma-helpers.c is already async. They just don't know >> >> >> that the memory access part is being made async too :). >> >> > >> >> > Can you point me to some info on that ? >> >> >> >> IDE and SCSI use dma-helpers.c to perform I/O: >> >> hw/ide/core.c:892: s->bus->dma->aiocb = >> >> dma_blk_io(blk_get_aio_context(s->blk), >> >> hw/ide/macio.c:189: s->bus->dma->aiocb = >> >> dma_blk_io(blk_get_aio_context(s->blk), &s->sg, >> >> hw/scsi/scsi-disk.c:348: r->req.aiocb = >> >> dma_blk_io(blk_get_aio_context(s->qdev.conf.blk), >> >> hw/scsi/scsi-disk.c:551: r->req.aiocb = >> >> dma_blk_io(blk_get_aio_context(s->qdev.conf.blk), >> >> >> >> They pass a scatter-gather list of guest RAM addresses to >> >> dma-helpers.c. They receive a callback when I/O has finished. >> >> >> >> Try following the code path. Request submission may be from a vcpu >> >> thread or IOThread. Completion occurs in the main loop or an >> >> IOThread. >> >> >> >> The main point is that this API is already asynchronous. If any >> >> changes are needed for async guest memory access (not sure, I haven't >> >> checked), then at least the dma-helpers.c users do not need to be >> >> modified. >> >> >> >> >> The remaining cases are virtio and some other devices. >> >> >> >> >> >> If you are worried about performance, the first rule is that async >> >> >> memory access is only needed on the destination side when post-copy is >> >> >> active. Maybe use setjmp to return from the signal handler and queue >> >> >> a callback for when the page has been loaded. >> >> > >> >> > I'm not sure it's worth trying to be too clever at avoiding this; >> >> > I see the fact that we're doing IO with the bql held as a more >> >> > fundamental problem. >> >> >> >> QEMU should be doing I/O syscalls in async fashion or threadpool >> >> workers (no BQL) so the BQL is not an issue. Anything else could >> >> cause unbounded waits even without postcopy. >> > >> > E.g. when vcpu got page faulted with BQL taken, while the main thread >> > needs the BQL to dispatch anything, including monitor commands. >> > >> > So I think it's a multiplex problem - we need to solve both (1) main >> > thread accessing guest memories which is still missing, and (2) BQL >> > deadlocks between vcpu threads and main thread. >> >> I think we need a single solution and cannot treat these as separate. >> This is because the same virtio device emulation code may run in 3 >> contexts: >> 1. vcpu thread (ioeventfd=off) >> 2. main loop thread (ioeventfd=on) >> 3. IOThread (ioeventfd=on, iothread=) >> >> If you try to solve them separately then the code won't work in all 3 >> contexts anymore. > > I think you can also get main loop thread hangs on things like > network packet reception. That is case #2. The QEMU net subsystem reads receive packets into a temporary buffer (it's not zero-copy) and invokes the virtio-net receive handler function from the main loop. Stefan