On Mon, Nov 26, 2018 at 01:03:46PM -0800, David Miller wrote:
>
> On T4 and later sparc64 cpus we can use the fused compare and branch
> instruction.
>
> However, it can only be used if the branch destination is in the range
> of a signed 10-bit immediate offset. This amounts to 1024
> instructions forwards or backwards.
>
> After the commit referenced in the Fixes: tag, the largest possible
> size program seen by the JIT explodes by a significant factor.
>
> As a result of this convergance takes many more passes since the
> expanded "BPF_LDX | BPF_MSH | BPF_B" code sequence, for example,
> contains several embedded branch on condition instructions.
>
> On each pass, as suddenly new fused compare and branch instances
> become valid, this makes thousands more in range for the next pass.
> And so on and so forth.
>
> This is most greatly exemplified by "BPF_MAXINSNS: exec all MSH" which
> takes 35 passes to converge, and shrinks the image by about 64K.
>
> To decrease the cost of this number of convergance passes, do the
> convergance pass before we have the program image allocated, just like
> other JITs (such as x86) do.
>
> Fixes: e0cea7ce988c ("bpf: implement ld_abs/ld_ind in native bpf")
> Signed-off-by: David S. Miller <da...@davemloft.net>
argh. nice catch.
Applied to bpf tree.
