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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id AC16BC4332F for ; Wed, 23 Nov 2022 10:51:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237151AbiKWKvM (ORCPT ); Wed, 23 Nov 2022 05:51:12 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51854 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236502AbiKWKue (ORCPT ); Wed, 23 Nov 2022 05:50:34 -0500 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 54B0212297E for ; Wed, 23 Nov 2022 02:36:37 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=MIME-Version:Content-Transfer-Encoding: Content-Type:References:In-Reply-To:Date:Cc:To:From:Subject:Message-ID:Sender :Reply-To:Content-ID:Content-Description; bh=/tHbzbSasw+hImnptkkp3nuEh8k6WTnyh4/3rgiCqtg=; b=F36by0QJC/hGNAId6GNID4dp3a cWDdL40++hOniXPaks3p3FXQRsROBS+Y7MrYxwAIOC0BZjIJmhKXdCliu7mHxlr30R3cOtYghPsRc cWX60BX99nR6sqks/FbNNEl2/6SztwhkEjU8d+8pGOAT2vEDUvzSjgvZ9UDUIWbSBqJM1Rs/XHqMv FN40HDSsDZAR+kF9CBnu61IEqmbXayjkdTZY5WgdYewZ6mRE0SojCRJHAsw81ubSJ2vIyBrEDryR6 NTOoLkGVV7VekuLbhpeNFVaKZuu/IfqWZ/BQX03iDkLnIIvBG0hN9e/S9KWWKAC/RuL3dQ9+QRWWo 9UMtjcEA==; Received: from [54.239.6.187] (helo=[192.168.3.58]) by casper.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1oxn7D-007RIP-51; Wed, 23 Nov 2022 10:36:27 +0000 Message-ID: <76dabea8f5a305870933bab27fa99a5fb6be3f6d.camel@infradead.org> Subject: Re: [PATCH v2] virtio_console: Introduce an ID allocator for virtual console numbers From: Amit Shah To: =?ISO-8859-1?Q?C=E9dric?= Le Goater , Amit Shah Cc: Arnd Bergmann , Greg Kroah-Hartman , virtualization@lists.linux-foundation.org, linux-kernel@vger.kernel.org, Thomas Huth , "Michael S. Tsirkin" Date: Wed, 23 Nov 2022 11:36:19 +0100 In-Reply-To: <62cfdcff-9323-5007-67e2-54178e43b21e@kaod.org> References: <20221114173811.1977367-1-clg@kaod.org> <6293fd7d415950979c55c3f09254a6f52eba708b.camel@infradead.org> <62cfdcff-9323-5007-67e2-54178e43b21e@kaod.org> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.44.4-0ubuntu1 MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 2022-11-23 at 11:17 +0100, C=C3=A9dric Le Goater wrote: > Hello Amit, >=20 > On 11/22/22 18:03, Amit Shah wrote: > > On Mon, 2022-11-14 at 18:38 +0100, C=C3=A9dric Le Goater wrote: > > > When a virtio console port is initialized, it is registered as an hvc > > > console using a virtual console number. If a KVM guest is started wit= h > > > multiple virtio console devices, the same vtermno (or virtual console > > > number) can be used to allocate different hvc consoles, which leads t= o > > > various communication problems later on. > > >=20 > > > This is also reported in debugfs : > > >=20 > > > # grep vtermno /sys/kernel/debug/virtio-ports/* > > > /sys/kernel/debug/virtio-ports/vport1p1:console_vtermno: 1 > > > /sys/kernel/debug/virtio-ports/vport2p1:console_vtermno: 1 > > > /sys/kernel/debug/virtio-ports/vport3p1:console_vtermno: 2 > > > /sys/kernel/debug/virtio-ports/vport4p1:console_vtermno: 3 > > >=20 > > > Replace the next_vtermno global with an ID allocator and start the > > > allocation at 1 as it is today. Also recycle IDs when a console port > > > is removed. > >=20 > > When the original virtio_console module was written, it didn't have > > support for multiple ports to be used this way. So the oddity you're > > seeing is left there deliberately: VMMs should not be instantiating > > console ports this way. > >=20 > > I don't know if we should take in this change, but can you walk through > > all combinations of new/old guest and new/old hypervisor and ensure > > nothing's going to break -- and confirm with the spec this is still OK > > to do? It may not be a goal to still ensure launches of a new guest on > > a very old (say) Centos5 guest still works -- but that was the point of > > maintaining backward compat... >=20 > 'next_vtermno' was introduced by d8a02bd58ab6 ("virtio: console: > remove global var") to differentiate the underlying kernel hvc console > associated with each virtio console port. Some drivers, like XEN, > simply use a magic/cookie number for instance. >=20 > This number is not related to the virtio specs. It is not exposed to > QEMU nor the guest (a part from debugfs). It's an internal identifier > related to the implementation in the kernel. I don't understand how > this could break compatibility. The change even keeps the allocated > range the same in case some assumption is made on vtermno 0. Am I > missing something ? No, you're right about this being kernel-internal -- just that it's used with hvc instead of qemu like I mentioned. I think this is the right change; just want to confirm hvc didn't get confused. >=20 > In the virtio console driver case, we could also generate a unique > number from the tuple { virtio device index, virtio console port }. > The ID allocator approach is simpler. I think the bug is that we don't increment the vtermno today in all places that we should; but this patch solves it too - I don't mind adding the extra ida bits.