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Tsirkin" To: Si-Wei Liu Cc: Parav Pandit , "netdev@vger.kernel.org" , "virtualization@lists.linux-foundation.org" , "xieyongji@bytedance.com" , "gautam.dawar@amd.com" , "Zhu, Lingshan" Subject: Re: [PATCH V3 5/6] vDPA: answer num of queue pairs = 1 to userspace when VIRTIO_NET_F_MQ == 0 Message-ID: <20220727050102-mutt-send-email-mst@kernel.org> References: <1246d2f1-2822-0edb-cd57-efc4015f05a2@intel.com> <19681358-fc81-be5b-c20b-7394a549f0be@intel.com> <20220727015626-mutt-send-email-mst@kernel.org> <66291287-fcb3-be8d-2c1b-dd44533ed8b3@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <66291287-fcb3-be8d-2c1b-dd44533ed8b3@oracle.com> Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Wed, Jul 27, 2022 at 12:50:33AM -0700, Si-Wei Liu wrote: > > > On 7/26/2022 11:01 PM, Michael S. Tsirkin wrote: > > On Wed, Jul 27, 2022 at 03:47:35AM +0000, Parav Pandit wrote: > > > > From: Zhu, Lingshan > > > > Sent: Tuesday, July 26, 2022 10:53 PM > > > > > > > > On 7/27/2022 10:17 AM, Parav Pandit wrote: > > > > > > From: Zhu, Lingshan > > > > > > Sent: Tuesday, July 26, 2022 10:15 PM > > > > > > > > > > > > On 7/26/2022 11:56 PM, Parav Pandit wrote: > > > > > > > > From: Zhu, Lingshan > > > > > > > > Sent: Tuesday, July 12, 2022 11:46 PM > > > > > > > > > When the user space which invokes netlink commands, detects that > > > > > > _MQ > > > > > > > > is not supported, hence it takes max_queue_pair = 1 by itself. > > > > > > > > I think the kernel module have all necessary information and it is > > > > > > > > the only one which have precise information of a device, so it > > > > > > > > should answer precisely than let the user space guess. The kernel > > > > > > > > module should be reliable than stay silent, leave the question to > > > > > > > > the user space > > > > > > tool. > > > > > > > Kernel is reliable. It doesn’t expose a config space field if the > > > > > > > field doesn’t > > > > > > exist regardless of field should have default or no default. > > > > > > so when you know it is one queue pair, you should answer one, not try > > > > > > to guess. > > > > > > > User space should not guess either. User space gets to see if _MQ > > > > > > present/not present. If _MQ present than get reliable data from kernel. > > > > > > > If _MQ not present, it means this device has one VQ pair. > > > > > > it is still a guess, right? And all user space tools implemented this > > > > > > feature need to guess > > > > > No. it is not a guess. > > > > > It is explicitly checking the _MQ feature and deriving the value. > > > > > The code you proposed will be present in the user space. > > > > > It will be uniform for _MQ and 10 other features that are present now and > > > > in the future. > > > > MQ and other features like RSS are different. If there is no _RSS_XX, there > > > > are no attributes like max_rss_key_size, and there is not a default value. > > > > But for MQ, we know it has to be 1 wihtout _MQ. > > > "we" = user space. > > > To keep the consistency among all the config space fields. > > Actually I looked and the code some more and I'm puzzled: > > > > > > struct virtio_net_config config = {}; > > u64 features; > > u16 val_u16; > > > > vdpa_get_config_unlocked(vdev, 0, &config, sizeof(config)); > > > > if (nla_put(msg, VDPA_ATTR_DEV_NET_CFG_MACADDR, sizeof(config.mac), > > config.mac)) > > return -EMSGSIZE; > > > > > > Mac returned even without VIRTIO_NET_F_MAC > > > > > > val_u16 = le16_to_cpu(config.status); > > if (nla_put_u16(msg, VDPA_ATTR_DEV_NET_STATUS, val_u16)) > > return -EMSGSIZE; > > > > > > status returned even without VIRTIO_NET_F_STATUS > > > > val_u16 = le16_to_cpu(config.mtu); > > if (nla_put_u16(msg, VDPA_ATTR_DEV_NET_CFG_MTU, val_u16)) > > return -EMSGSIZE; > > > > > > MTU returned even without VIRTIO_NET_F_MTU > > > > > > What's going on here? > > > > > I guess this is spec thing (historical debt), I vaguely recall these fields > are always present in config space regardless the existence of corresponding > feature bit. > > -Siwei Nope: 2.5.1 Driver Requirements: Device Configuration Space ... For optional configuration space fields, the driver MUST check that the corresponding feature is offered before accessing that part of the configuration space. -- MST