On 11/07, Mina Almasry wrote: > On Mon, Nov 6, 2023 at 5:06 PM Stanislav Fomichev <sdf@xxxxxxxxxx> wrote: > [..] > > > > > And the socket has to know this association; otherwise those tokens > > > > > are useless since they don't carry anything to identify the dmabuf. > > > > > > > > > > I think my other issue with MSG_SOCK_DEVMEM being on recvmsg is that > > > > > it somehow implies that I have an option of passing or not passing it > > > > > for an individual system call. > > > > > > You do have the option of passing it or not passing it per system > > > call. The MSG_SOCK_DEVMEM says the application is willing to receive > > > devmem cmsgs - that's all. The application doesn't get to decide > > > whether it's actually going to receive a devmem cmsg or not, because > > > that's dictated by the type of skb that is present in the receive > > > queue, and not up to the application. I should explain this in the > > > commit message... > > > > What would be the case of passing it or not passing it? Some fallback to > > the host memory after flow steering update? Yeah, would be useful to > > document those constrains. I'd lean on starting stricter and relaxing > > those conditions if we find the use-cases. > > > > MSG_SOCK_DEVMEM (or its replacement SOCK_DEVMEM or SO_SOCK_DEVMEM), > just says that the application is able to receive devmem cmsgs and > will parse them. The use case for not setting that flag is existing > applications that are not aware of devmem cmsgs. I don't want those > applications to think they're receiving data in the linear buffer only > to find out that the data is in devmem and they ignored the devmem > cmsg. > > So, what happens: > > - MSG_SOCK_DEVMEM provided and next skb in the queue is devmem: > application receives cmsgs. > - MSG_SOCK_DEVMEM provided and next skb in the queue is non-devmem: > application receives in the linear buffer. > - MSG_SOCK_DEVMEM not provided and net skb is devmem: application > receives EFAULT. > - MSG_SOCK_DEVMEM not provided and next skb is non-devmem: application > receives in the linear buffer. > > My bad on not including some docs about this. The next version should > have the commit message beefed up to explain all this, or a docs > patch. I don't understand. We require an elaborate setup to receive devmem cmsgs, why would some random application receive those?
