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[79.17.248.175]) by smtp.gmail.com with ESMTPSA id n10sm3361269wrv.77.2020.12.01.05.45.20 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 01 Dec 2020 05:45:21 -0800 (PST) Date: Tue, 1 Dec 2020 14:45:18 +0100 From: Stefano Garzarella To: Stefan Hajnoczi Cc: "Michael S. Tsirkin" , Stefan Hajnoczi , Mike Christie , fam , linux-scsi , Jason Wang , qemu-devel , Linux Virtualization , target-devel , Paolo Bonzini Subject: Re: [PATCH 00/10] vhost/qemu: thread per IO SCSI vq Message-ID: <20201201134518.pwrggkmixpyro4sg@steredhat> References: <20201118113117.GF182763@stefanha-x1.localdomain> <20201119094315-mutt-send-email-mst@kernel.org> <20201120072802-mutt-send-email-mst@kernel.org> <20201201125943.GE585157@stefanha-x1.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: <20201201125943.GE585157@stefanha-x1.localdomain> Precedence: bulk List-ID: X-Mailing-List: target-devel@vger.kernel.org Archived-At: List-Archive: List-Post: On Tue, Dec 01, 2020 at 12:59:43PM +0000, Stefan Hajnoczi wrote: >On Fri, Nov 20, 2020 at 07:31:08AM -0500, Michael S. Tsirkin wrote: >> On Fri, Nov 20, 2020 at 08:45:49AM +0000, Stefan Hajnoczi wrote: >> > On Thu, Nov 19, 2020 at 5:08 PM Stefan Hajnoczi wrote: >> > > >> > > On Thu, Nov 19, 2020 at 4:43 PM Mike Christie >> > > wrote: >> > > > >> > > > On 11/19/20 10:24 AM, Stefan Hajnoczi wrote: >> > > > > On Thu, Nov 19, 2020 at 4:13 PM Mike Christie >> > > > > wrote: >> > > > >> >> > > > >> On 11/19/20 8:46 AM, Michael S. Tsirkin wrote: >> > > > >>> On Wed, Nov 18, 2020 at 11:31:17AM +0000, Stefan Hajnoczi wrote: >> > > > > struct vhost_run_worker_info { >> > > > > struct timespec *timeout; >> > > > > sigset_t *sigmask; >> > > > > >> > > > > /* List of virtqueues to process */ >> > > > > unsigned nvqs; >> > > > > unsigned vqs[]; >> > > > > }; >> > > > > >> > > > > /* This blocks until the timeout is reached, a signal is received, or >> > > > > the vhost device is destroyed */ >> > > > > int ret = ioctl(vhost_fd, VHOST_RUN_WORKER, &info); >> > > > > >> > > > > As you can see, userspace isn't involved with dealing with the >> > > > > requests. It just acts as a thread donor to the vhost driver. >> > > > > >> > > > > We would want the VHOST_RUN_WORKER calls to be infrequent to avoid the >> > > > > penalty of switching into the kernel, copying in the arguments, etc. >> > > > >> > > > I didn't get this part. Why have the timeout? When the timeout expires, >> > > > does userspace just call right back down to the kernel or does it do >> > > > some sort of processing/operation? >> > > > >> > > > You could have your worker function run from that ioctl wait for a >> > > > signal or a wake up call from the vhost_work/poll functions. >> > > >> > > An optional timeout argument is common in blocking interfaces like >> > > poll(2), recvmmsg(2), etc. >> > > >> > > Although something can send a signal to the thread instead, >> > > implementing that in an application is more awkward than passing a >> > > struct timespec. >> > > >> > > Compared to other blocking calls we don't expect >> > > ioctl(VHOST_RUN_WORKER) to return soon, so maybe the timeout will >> > > rarely be used and can be dropped from the interface. >> > > >> > > BTW the code I posted wasn't a carefully thought out proposal :). The >> > > details still need to be considered and I'm going to be offline for >> > > the next week so maybe someone else can think it through in the >> > > meantime. >> > >> > One final thought before I'm offline for a week. If >> > ioctl(VHOST_RUN_WORKER) is specific to a single vhost device instance >> > then it's hard to support poll-mode (busy waiting) workers because >> > each device instance consumes a whole CPU. If we stick to an interface >> > where the kernel manages the worker threads then it's easier to share >> > workers between devices for polling. >> >> >> Yes that is the reason vhost did its own reason in the first place. >> >> >> I am vaguely thinking about poll(2) or a similar interface, >> which can wait for an event on multiple FDs. > >I can imagine how using poll(2) would work from a userspace perspective, >but on the kernel side I don't think it can be implemented cleanly. >poll(2) is tied to the file_operations->poll() callback and >read/write/error events. Not to mention there isn't a way to substitue >the vhost worker thread function instead of scheduling out the current >thread while waiting for poll fd events. > >But maybe ioctl(VHOST_WORKER_RUN) can do it: > > struct vhost_run_worker_dev { > int vhostfd; /* /dev/vhost-TYPE fd */ > unsigned nvqs; /* number of virtqueues in vqs[] */ > unsigned vqs[]; /* virtqueues to process */ > }; > > struct vhost_run_worker_info { > struct timespec *timeout; > sigset_t *sigmask; > > unsigned ndevices; > struct vhost_run_worker_dev *devices[]; > }; > >In the simple case userspace sets ndevices to 1 and we just handle >virtqueues for the current device. > >In the fancier shared worker thread case the userspace process has the >vhost fds of all the devices it is processing and passes them to >ioctl(VHOST_WORKER_RUN) via struct vhost_run_worker_dev elements. Which fd will be used for this IOCTL? One of the 'vhostfd' or we should create a new /dev/vhost-workers (or something similar)? Maybe the new device will be cleaner and can be reused also for other stuff (I'm thinking about vDPA software devices). > >>From a security perspective it means the userspace thread has access to >all vhost devices (because it has their fds). > >I'm not sure how the mm is supposed to work. The devices might be >associated with different userspace processes (guests) and therefore >have different virtual memory. Maybe in this case we should do something similar to io_uring SQPOLL kthread where kthread_use_mm()/kthread_unuse_mm() is used to switch virtual memory spaces. After writing, I saw that we already do it this in the vhost_worker() in drivers/vhost/vhost.c > >Just wanted to push this discussion along a little further. I'm buried >under emails and probably wont be very active over the next few days. > I think ioctl(VHOST_WORKER_RUN) might be the right way and also maybe the least difficult one. Thanks, Stefano