netdev - Re: [PATCH bpf-next v2 0/9] riscv: BPF JIT fix, optimizations and far jumps support

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Message-ID: <20191219150726.GA23959@pc-9.home>
Date:   Thu, 19 Dec 2019 16:07:26 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Björn Töpel <bjorn.topel@...il.com>
Cc:     ast@...nel.org, netdev@...r.kernel.org,
        linux-riscv@...ts.infradead.org, bpf@...r.kernel.org
Subject: Re: [PATCH bpf-next v2 0/9] riscv: BPF JIT fix, optimizations and
 far jumps support

On Mon, Dec 16, 2019 at 10:13:34AM +0100, Björn Töpel wrote:
> 
> This series contain one non-critical fix, support for far jumps. and
> some optimizations for the BPF JIT.
> 
> Previously, the JIT only supported 12b branch targets for conditional
> branches, and 21b for unconditional branches. Starting with this
> series, 32b branching is supported.
> 
> As part of supporting far jumps, branch relaxation was introduced. The
> idea is to start with a pessimistic jump (e.g. auipc/jalr) and for
> each pass the JIT will have an opportunity to pick a better
> instruction (e.g. jal) and shrink the image. Instead of two passes,
> the JIT requires more passes. It typically converges after 3 passes.
> 
> The optimizations mentioned in the subject are for calls and tail
> calls. In the tail call generation we can save one instruction by
> using the offset in jalr. Calls are optimized by doing (auipc)/jal(r)
> relative jumps instead of loading the entire absolute address and
> doing jalr. This required that the JIT image allocator was made RISC-V
> specific, so we can ensure that the JIT image and the kernel text are
> in range (32b).
> 
> The last two patches of the series is not critical to the series, but
> are two UAPI build issues for BPF events. A closer look from the
> RV-folks would be much appreciated.
> 
> The test_bpf.ko module, selftests/bpf/test_verifier and
> selftests/seccomp/seccomp_bpf pass all tests.
> 
> RISC-V is still missing proper kprobe and tracepoint support, so a lot
> of BPF selftests cannot be run.

Applied, thanks!