From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:56274) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1c3ppR-0007Nm-68 for qemu-devel@nongnu.org; Mon, 07 Nov 2016 14:43:38 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1c3ppN-0001gH-As for qemu-devel@nongnu.org; Mon, 07 Nov 2016 14:43:37 -0500 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:55305) by eggs.gnu.org with esmtps (TLS1.0:RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1c3ppN-0001fu-0b for qemu-devel@nongnu.org; Mon, 07 Nov 2016 14:43:33 -0500 Received: from pps.filterd (m0098404.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.17/8.16.0.17) with SMTP id uA7JcbY1016766 for ; Mon, 7 Nov 2016 14:43:31 -0500 Received: from e06smtp10.uk.ibm.com (e06smtp10.uk.ibm.com [195.75.94.106]) by mx0a-001b2d01.pphosted.com with ESMTP id 26jvt7yqfs-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Mon, 07 Nov 2016 14:43:31 -0500 Received: from localhost by e06smtp10.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Mon, 7 Nov 2016 19:43:29 -0000 References: <1478257275-30365-1-git-send-email-mst@redhat.com> <1478257275-30365-3-git-send-email-mst@redhat.com> <20161107153853.GF2054@work-vm> From: Halil Pasic Date: Mon, 7 Nov 2016 20:43:26 +0100 MIME-Version: 1.0 In-Reply-To: <20161107153853.GF2054@work-vm> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 8bit Message-Id: Subject: Re: [Qemu-devel] [PATCH 2/2] virtio-net: mark VIRTIO_NET_F_GSO as legacy List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Dr. David Alan Gilbert" , "Michael S. Tsirkin" Cc: Jason Wang , qemu-devel@nongnu.org, qemu-stable@nongnu.org On 11/07/2016 04:38 PM, Dr. David Alan Gilbert wrote: > * Michael S. Tsirkin (mst@redhat.com) wrote: >> > virtio 1.0 spec says this is a legacy feature bit, >> > hide it from guests in legacy mode. >> > >> > Note: for cross-version migration compatibility, >> > we keep the bit set in host_features. >> > The result will be that a guest migrating cross-version >> > will see host features change under it. >> > As guests only seem to read it once, this should >> > not be an issue. Meanwhile, will work to fix guests to >> > ignore this bit in virtio1 mode, too. > OK, but if they're actually using the feature they'll carry > on working ? > > Dave > The interesting scenario is when we migrate form older to newer (otherwise the bugous situation can't emerge in the first place). Now the change affects only what the guest sees when it reads host_features, and that should happen only during the feature negotiation (once). That means the change won't have any observable effect unless a reset happens. And if we have a reset, then things are going to work out correctly avoiding the bugous (but still working) state in both host and guest. So yes, both will happily carry on using the feature, and carry on working. Halil