netdev - Re: [PATCH bpf-next 1/4] xdp: Support specifying expected existing program when attaching XDP

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Date:   Fri, 20 Mar 2020 14:30:14 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Toke Høiland-Jørgensen <toke@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        "David S. Miller" <davem@...emloft.net>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        John Fastabend <john.fastabend@...il.com>,
        Lorenz Bauer <lmb@...udflare.com>,
        Andrey Ignatov <rdna@...com>, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Subject: Re: [PATCH bpf-next 1/4] xdp: Support specifying expected existing
 program when attaching XDP

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@...nel.org> writes:  
> >>> On Thu, 19 Mar 2020 14:13:13 +0100 Toke Høiland-Jørgensen wrote:  
> >>>> From: Toke Høiland-Jørgensen <toke@...hat.com>
> >>>>
> >>>> 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@...hat.com>  
> >>>
> >>> 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