From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id B0711C63777 for ; Fri, 20 Nov 2020 12:31:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0BFB02087D for ; Fri, 20 Nov 2020 12:31:19 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="Oeg+aEXh" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727043AbgKTMbT (ORCPT ); Fri, 20 Nov 2020 07:31:19 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:27833 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726559AbgKTMbT (ORCPT ); Fri, 20 Nov 2020 07:31:19 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1605875477; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=ocK86ENwpJGmMxdlo0mfB3bMPeCsw3DadrlnqYb0ooI=; b=Oeg+aEXhkgRd0O1/JSAl2iysutQ2D0m6qje6XJhWkhRu22QqDXfq6Nsz5u04lko12fRJLE kZJrHlyILQDqc+aEvZZ/gV3cThdAmjUjlRRIW/EeJOI8b7AM2tdRKpppWU6By7AxQqrVB1 nV8MX2bE3+TMWmUqV+pMjW8XUVpimG8= Received: from mail-wm1-f72.google.com (mail-wm1-f72.google.com [209.85.128.72]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-341-c5IJV4NvMl2MgPobeuX0QQ-1; Fri, 20 Nov 2020 07:31:15 -0500 X-MC-Unique: c5IJV4NvMl2MgPobeuX0QQ-1 Received: by mail-wm1-f72.google.com with SMTP id o19so3992751wme.2 for ; Fri, 20 Nov 2020 04:31:15 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=ocK86ENwpJGmMxdlo0mfB3bMPeCsw3DadrlnqYb0ooI=; b=lTpstEvI96owLbh0tswSMjrgCGOUQO91mY/QelkxZPXfeZHwkxta1MANAtsuB+m1vD 1sZIC3MfXVV1dfM5B4+YjN9VxVlYgqFgYgTybIh8PoArzmMeugZVzsVRJ+OW+RbdHGQ3 Ivho/fVisuIEb9f3piXEWss5KLPCX+S0IyrNTeHH8o9JpVV+ygW598rFAhQyJ2GUQCSe fRMeaj1fuU3FRa8YkfXx+30Xwq6REKzsFcwvr2IzGAB6jPrn1xt3LWj0r5BMxDdoEmYJ JcKRHe13DYJUzNaVo4gaXB5S+06uCdCWMEydJ2AHQuPHkeLwaEp4oCG5l6lokZQU9vQM b+zA== X-Gm-Message-State: AOAM532+U6Sq5+6Ft7RWnTowi98bE6nnvunUgaQbc/sl9zdIcPghSAx9 0ECXPEGDToCkvleTEN3gjlolBWXUWAw/SqK7kL0iEXjfR1yBvLUKoPTOdHivIg8k1ADxK9POR5X bfYFE2Kbl/vzkNMKu5LHDvQ== X-Received: by 2002:a5d:6a83:: with SMTP id s3mr15575451wru.397.1605875474261; Fri, 20 Nov 2020 04:31:14 -0800 (PST) X-Google-Smtp-Source: ABdhPJz5yeyXryHim+8cbbaVgxlvvJwdtVTGEJ7CX1LFkcQa+sotAPHYcwKeUvV6gKolUT2mX0mACQ== X-Received: by 2002:a5d:6a83:: with SMTP id s3mr15575426wru.397.1605875474024; Fri, 20 Nov 2020 04:31:14 -0800 (PST) Received: from redhat.com (bzq-79-176-44-197.red.bezeqint.net. [79.176.44.197]) by smtp.gmail.com with ESMTPSA id v6sm5115620wrb.53.2020.11.20.04.31.10 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 20 Nov 2020 04:31:12 -0800 (PST) Date: Fri, 20 Nov 2020 07:31:08 -0500 From: "Michael S. Tsirkin" To: Stefan Hajnoczi Cc: Mike Christie , Stefan Hajnoczi , fam , linux-scsi , Jason Wang , qemu-devel , Linux Virtualization , target-devel , Paolo Bonzini , Stefano Garzarella Subject: Re: [PATCH 00/10] vhost/qemu: thread per IO SCSI vq Message-ID: <20201120072802-mutt-send-email-mst@kernel.org> References: <1605223150-10888-1-git-send-email-michael.christie@oracle.com> <20201117164043.GS131917@stefanha-x1.localdomain> <20201118113117.GF182763@stefanha-x1.localdomain> <20201119094315-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-scsi@vger.kernel.org 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 have CCed Stefano Garzarella, who is looking at similar designs for > vDPA software device implementations. > > Stefan From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-3.5 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 79C01C5519F for ; Fri, 20 Nov 2020 12:31:24 +0000 (UTC) Received: from whitealder.osuosl.org (smtp1.osuosl.org [140.211.166.138]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id E13B82087D for ; 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[79.176.44.197]) by smtp.gmail.com with ESMTPSA id v6sm5115620wrb.53.2020.11.20.04.31.10 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 20 Nov 2020 04:31:12 -0800 (PST) Date: Fri, 20 Nov 2020 07:31:08 -0500 From: "Michael S. Tsirkin" To: Stefan Hajnoczi Subject: Re: [PATCH 00/10] vhost/qemu: thread per IO SCSI vq Message-ID: <20201120072802-mutt-send-email-mst@kernel.org> References: <1605223150-10888-1-git-send-email-michael.christie@oracle.com> <20201117164043.GS131917@stefanha-x1.localdomain> <20201118113117.GF182763@stefanha-x1.localdomain> <20201119094315-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 In-Reply-To: Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=mst@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Disposition: inline Cc: fam , linux-scsi , qemu-devel , Linux Virtualization , target-devel , Stefan Hajnoczi , Paolo Bonzini , Mike Christie X-BeenThere: virtualization@lists.linux-foundation.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: Linux virtualization List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: virtualization-bounces@lists.linux-foundation.org Sender: "Virtualization" 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 have CCed Stefano Garzarella, who is looking at similar designs for > vDPA software device implementations. > > Stefan _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-3.6 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 41B57C5519F for ; Fri, 20 Nov 2020 12:33:28 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 7CF672151B for ; Fri, 20 Nov 2020 12:33:27 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="Oeg+aEXh" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 7CF672151B Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:41238 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1kg5bS-00038J-D8 for qemu-devel@archiver.kernel.org; Fri, 20 Nov 2020 07:33:26 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:50532) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1kg5ZT-0001Oz-Ub for qemu-devel@nongnu.org; Fri, 20 Nov 2020 07:31:24 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:21912) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1kg5ZO-0004o3-63 for qemu-devel@nongnu.org; Fri, 20 Nov 2020 07:31:23 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1605875477; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=ocK86ENwpJGmMxdlo0mfB3bMPeCsw3DadrlnqYb0ooI=; b=Oeg+aEXhkgRd0O1/JSAl2iysutQ2D0m6qje6XJhWkhRu22QqDXfq6Nsz5u04lko12fRJLE kZJrHlyILQDqc+aEvZZ/gV3cThdAmjUjlRRIW/EeJOI8b7AM2tdRKpppWU6By7AxQqrVB1 nV8MX2bE3+TMWmUqV+pMjW8XUVpimG8= Received: from mail-wm1-f72.google.com (mail-wm1-f72.google.com [209.85.128.72]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-513-Kzn2HiYcOhiePEzipr6vpw-1; Fri, 20 Nov 2020 07:31:15 -0500 X-MC-Unique: Kzn2HiYcOhiePEzipr6vpw-1 Received: by mail-wm1-f72.google.com with SMTP id j62so3989813wma.4 for ; Fri, 20 Nov 2020 04:31:15 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=ocK86ENwpJGmMxdlo0mfB3bMPeCsw3DadrlnqYb0ooI=; b=HnVKwYp5cAM97hCK7+V3czg3ON/vtWGOzWC1BmoFNtmR9i2vbR0ONcPNH3bgnicP7V JlUJJY9QyXpnMpEZIVkbQcOXAnzIqSp9c29W8lXQ2YIYwRq5+Z7pElo2+lP8ZtkP6ojY 8qKPsVrNaScVezogu3SszsYaAbQg/4UyloM/CaIKnqpbldlq6LChZru3aOekOBA8Xxeq RQ6dsB1AS4tis4oSAENrMwiFj01XF74t8jXqL//8de5GCtmg/SuXqKrBAcKWMsimPtvv nK72zeToB8jJQwMvXENd2H4Ufjbbuk9WKZhAHL/rcqCA/8gN+NoXMfCgPL1aYkhhzZML Wigg== X-Gm-Message-State: AOAM5328QZBQTtic1xfldGJKwrycvi0MWTC+qS5IOu4fb6+C7e8D2hd6 qezsri8GFIidNdVELg5FpxEbxkrILQMyauoOZc5WNn9c281p02Fc09lgqc1BJ4oygwQm0mSbUyw /c62j5z2gTGkbBC0= X-Received: by 2002:a5d:6a83:: with SMTP id s3mr15575452wru.397.1605875474261; Fri, 20 Nov 2020 04:31:14 -0800 (PST) X-Google-Smtp-Source: ABdhPJz5yeyXryHim+8cbbaVgxlvvJwdtVTGEJ7CX1LFkcQa+sotAPHYcwKeUvV6gKolUT2mX0mACQ== X-Received: by 2002:a5d:6a83:: with SMTP id s3mr15575426wru.397.1605875474024; Fri, 20 Nov 2020 04:31:14 -0800 (PST) Received: from redhat.com (bzq-79-176-44-197.red.bezeqint.net. [79.176.44.197]) by smtp.gmail.com with ESMTPSA id v6sm5115620wrb.53.2020.11.20.04.31.10 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 20 Nov 2020 04:31:12 -0800 (PST) Date: Fri, 20 Nov 2020 07:31:08 -0500 From: "Michael S. Tsirkin" To: Stefan Hajnoczi Subject: Re: [PATCH 00/10] vhost/qemu: thread per IO SCSI vq Message-ID: <20201120072802-mutt-send-email-mst@kernel.org> References: <1605223150-10888-1-git-send-email-michael.christie@oracle.com> <20201117164043.GS131917@stefanha-x1.localdomain> <20201118113117.GF182763@stefanha-x1.localdomain> <20201119094315-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 In-Reply-To: Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=mst@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=216.205.24.124; envelope-from=mst@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: fam , linux-scsi , Jason Wang , qemu-devel , Linux Virtualization , target-devel , Stefan Hajnoczi , Paolo Bonzini , Mike Christie , Stefano Garzarella Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" 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 have CCed Stefano Garzarella, who is looking at similar designs for > vDPA software device implementations. > > Stefan