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 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 smtp.lore.kernel.org (Postfix) with ESMTPS id DFE54C7EE22 for ; Thu, 11 May 2023 20:13:22 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1pxCe7-0004pQ-AV; Thu, 11 May 2023 16:12:15 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1pxCe6-0004pA-Go for qemu-devel@nongnu.org; Thu, 11 May 2023 16:12:14 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1pxCe4-0005UI-Ok for qemu-devel@nongnu.org; Thu, 11 May 2023 16:12:14 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1683835931; 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=T2V413X5+i82I8ibkxixPdg5TkN8XN0+FCl19AWIH/c=; b=JSiL4QWraChCiaFjlDcCqOR9LZ7rwGkC+zXr2tVFTE9XD/rSPYkIibNyyYMHk7CFr9OT8I cV7uzfZXmQDlSmq7V018nnJFboiA/3dZOxDB0p/e9ZQgeWy5p6EVEPL0D64Q7y8D9k9OzZ mvNqk9xY+Y28ue3KTvN4938DmBV5/sA= Received: from mimecast-mx02.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-302-MgAbwRbGPoSDIs3I0ntU4g-1; Thu, 11 May 2023 16:12:07 -0400 X-MC-Unique: MgAbwRbGPoSDIs3I0ntU4g-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.rdu2.redhat.com [10.11.54.8]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id E91CB29DD9A1; Thu, 11 May 2023 20:12:06 +0000 (UTC) Received: from localhost (unknown [10.39.194.137]) by smtp.corp.redhat.com (Postfix) with ESMTP id 9072FC15BA0; Thu, 11 May 2023 20:12:05 +0000 (UTC) Date: Wed, 10 May 2023 17:40:00 -0400 From: Stefan Hajnoczi To: Kevin Wolf Cc: qemu-devel@nongnu.org, Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= , Juan Quintela , Julia Suvorova , xen-devel@lists.xenproject.org, eesposit@redhat.com, Richard Henderson , Fam Zheng , "Michael S. Tsirkin" , Coiby Xu , David Woodhouse , Marcel Apfelbaum , Peter Lieven , Paul Durrant , "Richard W.M. Jones" , qemu-block@nongnu.org, Stefano Garzarella , Anthony Perard , Stefan Weil , Xie Yongji , Paolo Bonzini , Aarushi Mehta , Philippe =?iso-8859-1?Q?Mathieu-Daud=E9?= , Eduardo Habkost , Stefano Stabellini , Hanna Reitz , Ronnie Sahlberg Subject: Re: [PATCH v4 16/20] virtio: make it possible to detach host notifier from any thread Message-ID: <20230510214000.GC1287730@fedora> References: <20230425172716.1033562-1-stefanha@redhat.com> <20230425172716.1033562-17-stefanha@redhat.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="5oxBC66SYAo2b05z" Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.1 on 10.11.54.8 Received-SPF: pass client-ip=170.10.133.124; envelope-from=stefanha@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -10 X-Spam_score: -1.1 X-Spam_bar: - X-Spam_report: (-1.1 / 5.0 requ) BAYES_00=-1.9, DATE_IN_PAST_12_24=1.049, 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_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=no autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org --5oxBC66SYAo2b05z Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Thu, May 04, 2023 at 11:00:35PM +0200, Kevin Wolf wrote: > Am 25.04.2023 um 19:27 hat Stefan Hajnoczi geschrieben: > > virtio_queue_aio_detach_host_notifier() does two things: > > 1. It removes the fd handler from the event loop. > > 2. It processes the virtqueue one last time. > >=20 > > The first step can be peformed by any thread and without taking the > > AioContext lock. > >=20 > > The second step may need the AioContext lock (depending on the device > > implementation) and runs in the thread where request processing takes > > place. virtio-blk and virtio-scsi therefore call > > virtio_queue_aio_detach_host_notifier() from a BH that is scheduled in > > AioContext > >=20 > > Scheduling a BH is undesirable for .drained_begin() functions. The next > > patch will introduce a .drained_begin() function that needs to call > > virtio_queue_aio_detach_host_notifier(). >=20 > Why is it undesirable? In my mental model, .drained_begin() is still > free to start as many asynchronous things as it likes. The only > important thing to take care of is that .drained_poll() returns true as > long as the BH (or other asynchronous operation) is still pending. >=20 > Of course, your way of doing things still seems to result in simpler > code because you don't have to deal with a BH at all if you only really > want the first part and not the second. I have clarified this in the commit description. We can't wait synchronously, but we could wait asynchronously as you described. It's simpler to split the function instead of implementing async wait using =2Edrained_poll(). >=20 > > Move the virtqueue processing out to the callers of > > virtio_queue_aio_detach_host_notifier() so that the function can be > > called from any thread. This is in preparation for the next patch. >=20 > Did you forget to remove it in virtio_queue_aio_detach_host_notifier()? > If it's unchanged, I don't think the AioContext requirement is lifted. Yes! Thank you :) >=20 > > Signed-off-by: Stefan Hajnoczi > > --- > > hw/block/dataplane/virtio-blk.c | 2 ++ > > hw/scsi/virtio-scsi-dataplane.c | 9 +++++++++ > > 2 files changed, 11 insertions(+) > > diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virti= o-blk.c > > index b28d81737e..bd7cc6e76b 100644 > > --- a/hw/block/dataplane/virtio-blk.c > > +++ b/hw/block/dataplane/virtio-blk.c > > @@ -286,8 +286,10 @@ static void virtio_blk_data_plane_stop_bh(void *op= aque) > > =20 > > for (i =3D 0; i < s->conf->num_queues; i++) { > > VirtQueue *vq =3D virtio_get_queue(s->vdev, i); > > + EventNotifier *host_notifier =3D virtio_queue_get_host_notifie= r(vq); > > =20 > > virtio_queue_aio_detach_host_notifier(vq, s->ctx); > > + virtio_queue_host_notifier_read(host_notifier); > > } > > } >=20 > The existing code in virtio_queue_aio_detach_host_notifier() has a > comment before the read: >=20 > /* Test and clear notifier before after disabling event, > * in case poll callback didn't have time to run. */ >=20 > Do we want to keep it around in the new places? (And also fix the > "before after", I suppose, or replace it with a similar, but better > comment that explains why we're reading here.) I will add the comment. Stefan --5oxBC66SYAo2b05z Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEhpWov9P5fNqsNXdanKSrs4Grc8gFAmRcDzAACgkQnKSrs4Gr c8i8mQf5AYxgNAuDvbOU1XvzA5Q6jeekOXOQm5LcsUhp7ukc76FAwjsIrpvmfZxR 8v6pkq1ReqvvojVK6qpC9HRB/WX94Ss+128FOa7TJ0pmD9jJHJbyHNEIg722GVk4 EdSE1Or7d97FsMOt5EqwJxQ8RtXgei/lpfyx4OhKXLLYQ+puTlrq8TW8O7wzBFDe ZR9Sr8wDH9/yseUnOl17U6fJa9cUYroFGg6oZifWuTOPZAxwwgnryMOylwsknfUg pSTtGXkph5Ugf+KOxdrG98eXR+MCiCs/DrnIUuy0RBf7jEqcqT4STNAqIVb8TJaa gXzKpleSUOXubjuUJPcjpkQOP5ytMQ== =8VSX -----END PGP SIGNATURE----- --5oxBC66SYAo2b05z--