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Tsirkin" , =?UTF-8?Q?Daniel_P=2e_Berrang=c3=a9?= References: <20210204202915.15925-1-yuri.benditovich@daynix.com> <20210209093201-mutt-send-email-mst@kernel.org> <20210209145105.GP1166421@redhat.com> <20210209095553-mutt-send-email-mst@kernel.org> From: Jason Wang Message-ID: <0890bb17-9677-ff1d-bd08-c9be791e1c81@redhat.com> Date: Wed, 10 Feb 2021 14:19:59 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20210209095553-mutt-send-email-mst@kernel.org> X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=jasowang@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Received-SPF: pass client-ip=63.128.21.124; envelope-from=jasowang@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -35 X-Spam_score: -3.6 X-Spam_bar: --- X-Spam_report: (-3.6 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.57, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.265, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=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: yan@daynix.com, Yuri Benditovich , qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 2021/2/9 下午11:04, Michael S. Tsirkin wrote: > On Tue, Feb 09, 2021 at 02:51:05PM +0000, Daniel P. Berrangé wrote: >> On Tue, Feb 09, 2021 at 09:34:20AM -0500, Michael S. Tsirkin wrote: >>> On Thu, Feb 04, 2021 at 10:29:12PM +0200, Yuri Benditovich wrote: >>>> This set of patches introduces graceful switch from tap-vhost to >>>> tap-no-vhost depending on guest features. Before that the features >>>> that vhost does not support were silently cleared in get_features. >>>> This creates potential problem of migration from the machine where >>>> some of virtio-net features are supported by the vhost kernel to the >>>> machine where they are not supported (packed ring as an example). >>> I still worry that adding new features will silently disable vhost for people. >>> Can we limit the change to when a VM is migrated in? >> Some management applications expect bi-directional live migration to >> work, so taking specific actions on incoming migration only feels >> dangerous. > Could you be more specific? > > Bi-directional migration is currently broken > when migrating new kernel->old kernel. > > This seems to be the motivation for this patch, though I wish > it was spelled out more explicitly. > > People don't complain much, but I'm fine with fixing that > with a userspace fallback. > > > I'd rather not force the fallback on others though: vhost is generally > specified explicitly by user while features are generally set > automatically, so this patch will make us override what user specified, > not nice. > > >> IMHO if the features we're adding cannot be expected to exist in >> host kernels in general, then the feature should defualt to off >> and require explicit user config to enable. >> Downstream distros which can guarantee newer kernels can flip the >> default in their custom machine types if they desire. >> >> Regards, >> Daniel > Unfortunately that will basically mean we are stuck with no new features > for years. We did what this patch is trying to change for years now, in > particular KVM also seems to happily disable CPU features not supported > by kernel so I wonder why we can't keep doing it, with tweaks for some > corner cases. It's probably not the corner case. So my understanding is when a feature is turned on via command line, it should not be cleared silently otherwise we may break migration for sure. E.g when packed=on is specified, we should disable vhost instead of clear it from the device. Thanks > > userspace and kernel not being in 100% sync wrt features is not > a corner case though, and switching backends seems like too big > a hammer. > >> -- >> |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| >> |: https://libvirt.org -o- https://fstop138.berrange.com :| >> |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :| >