Jakub Kicinski <kuba@xxxxxxxxxx> writes: > On Tue, 21 Sep 2021 18:06:35 +0200 Toke Høiland-Jørgensen wrote: >> 1. Do nothing. This would make it up to users / sysadmins to avoid >> anything breaking by manually making sure to not enable multi-buffer >> support while loading any XDP programs that will malfunction if >> presented with an mb frame. This will probably break in interesting >> ways, but it's nice and simple from an implementation PoV. With this >> we don't need the declaration discussed above either. >> >> 2. Add a check at runtime and drop the frames if they are mb-enabled and >> the program doesn't understand it. This is relatively simple to >> implement, but it also makes for difficult-to-understand issues (why >> are my packets suddenly being dropped?), and it will incur runtime >> overhead. >> >> 3. Reject loading of programs that are not MB-aware when running in an >> MB-enabled mode. This would make things break in more obvious ways, >> and still allow a userspace loader to declare a program "MB-aware" to >> force it to run if necessary. The problem then becomes at what level >> to block this? >> >> Doing this at the driver level is not enough: while a particular >> driver knows if it's running in multi-buff mode, we can't know for >> sure if a particular XDP program is multi-buff aware at attach time: >> it could be tail-calling other programs, or redirecting packets to >> another interface where it will be processed by a non-MB aware >> program. >> >> So another option is to make it a global toggle: e.g., create a new >> sysctl to enable multi-buffer. If this is set, reject loading any XDP >> program that doesn't support multi-buffer mode, and if it's unset, >> disable multi-buffer mode in all drivers. This will make it explicit >> when the multi-buffer mode is used, and prevent any accidental subtle >> malfunction of existing XDP programs. The drawback is that it's a >> mode switch, so more configuration complexity. > > 4. Add new program type, XDP_MB. Do not allow mixing of XDP vs XDP_MB > thru tail calls. > > IMHO that's very simple and covers majority of use cases. Using the program type (or maybe the expected_attach_type) was how I was imagining we'd encode the "I am MB aware" flag, yes. I hadn't actually considered that this could be used to also restrict tail call/freplace attachment, but that's a good point. So this leaves just the redirect issue, then, see my other reply. -Toke
