netdev - Re: [PATCH bpf] sparc: Fix JIT fused branch convergance.

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Message-ID: <20181127015029.xhlgjet5si2kxjwt@ast-mbp.dhcp.thefacebook.com>
Date:   Mon, 26 Nov 2018 17:50:31 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     David Miller <davem@...emloft.net>
Cc:     daniel@...earbox.net, ast@...nel.org, netdev@...r.kernel.org
Subject: Re: [PATCH bpf] sparc: Fix JIT fused branch convergance.

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 <davem@...emloft.net>

argh. nice catch.
Applied to bpf tree.