On Fri, 20 Mar 2020 21:40:46 +0100 Daniel Borkmann wrote: > On 3/20/20 9:30 PM, Daniel Borkmann wrote: > > On 3/20/20 9:48 AM, Toke Høiland-Jørgensen wrote: > >> Jakub Kicinski <kuba@xxxxxxxxxx> writes: > >>> On Thu, 19 Mar 2020 14:13:13 +0100 Toke Høiland-Jørgensen wrote: > >>>> From: Toke Høiland-Jørgensen <toke@xxxxxxxxxx> > >>>> > >>>> While it is currently possible for userspace to specify that an existing > >>>> XDP program should not be replaced when attaching to an interface, there is > >>>> no mechanism to safely replace a specific XDP program with another. > >>>> > >>>> This patch adds a new netlink attribute, IFLA_XDP_EXPECTED_FD, which can be > >>>> set along with IFLA_XDP_FD. If set, the kernel will check that the program > >>>> currently loaded on the interface matches the expected one, and fail the > >>>> operation if it does not. This corresponds to a 'cmpxchg' memory operation. > >>>> > >>>> A new companion flag, XDP_FLAGS_EXPECT_FD, is also added to explicitly > >>>> request checking of the EXPECTED_FD attribute. This is needed for userspace > >>>> to discover whether the kernel supports the new attribute. > >>>> > >>>> Signed-off-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx> > >>> > >>> I didn't know we wanted to go ahead with this... > >> > >> Well, I'm aware of the bpf_link discussion, obviously. Not sure what's > >> happening with that, though. So since this is a straight-forward > >> extension of the existing API, that doesn't carry a high implementation > >> cost, I figured I'd just go ahead with this. Doesn't mean we can't have > >> something similar in bpf_link as well, of course. > > > > Overall series looks okay, but before we go down that road, especially given there is > > the new bpf_link object now, I would like us to first elaborate and figure out how XDP > > fits into the bpf_link concept, where its limitations are, whether it even fits at all, > > and how its semantics should look like realistically given bpf_link is to be generic to > > all program types. Then we could extend the atomic replace there generically as well. I > > think at the very minimum it might have similarities with what is proposed here, but > > from a user experience I would like to avoid having something similar in XDP API and > > then again in bpf_link which would just be confusing.. > > Another aspect that falls into this atomic replacement is also that the programs can > actually be atomically replaced at runtime. Last time I looked, some drivers still do > a down/up cycle on replacement and hence traffic would be interrupted. I would argue > that such /atomic/ swap operation on bpf_link would cover a guarantee of not having to > perform this as well (workaround today would be a simple tail call map as entry point). I don't think that's the case. Drivers generally have a fast path for the active-active replace. Up/Down is only done to remap DMA buffers and change RX buffer allocation scheme. That's when program is installed or removed, not replaced. I'm sure bpf_link would have solved this problem, though, and all the other problems we don't actually have :-P
