linux-kernel - Re: [PATCH v1 06/10] powerpc/bpf: Perform complete extra passes to update addresses

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Date:   Tue, 13 Dec 2022 15:53:48 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>
Cc:     Andrii Nakryiko <andrii@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>, bpf@...r.kernel.org,
        Daniel Borkmann <daniel@...earbox.net>,
        Hao Luo <haoluo@...gle.com>,
        John Fastabend <john.fastabend@...il.com>,
        Jiri Olsa <jolsa@...nel.org>, KP Singh <kpsingh@...nel.org>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Stanislav Fomichev <sdf@...gle.com>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>
Subject: Re: [PATCH v1 06/10] powerpc/bpf: Perform complete extra passes to
 update addresses

Christophe Leroy wrote:
> BPF core calls the jit compiler again for an extra pass in order
> to properly set subprog addresses.
> 
> Unlike other architectures, powerpc only updates the addresses
> during that extra pass. It means that holes must have been left
> in the code in order to enable the maximum possible instruction
> size.
> 
> In order avoid waste of space, and waste of CPU time on powerpc
> processors on which the NOP instruction is not 0-cycle, perform
> two real additional passes.
> 
> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
> ---
>  arch/powerpc/net/bpf_jit_comp.c | 85 ---------------------------------
>  1 file changed, 85 deletions(-)
> 
> diff --git a/arch/powerpc/net/bpf_jit_comp.c b/arch/powerpc/net/bpf_jit_comp.c
> index 43e634126514..8833bf23f5aa 100644
> --- a/arch/powerpc/net/bpf_jit_comp.c
> +++ b/arch/powerpc/net/bpf_jit_comp.c
> @@ -23,74 +23,6 @@ static void bpf_jit_fill_ill_insns(void *area, unsigned int size)
>  	memset32(area, BREAKPOINT_INSTRUCTION, size / 4);
>  }
>  
> -/* Fix updated addresses (for subprog calls, ldimm64, et al) during extra pass */
> -static int bpf_jit_fixup_addresses(struct bpf_prog *fp, u32 *image,
> -				   struct codegen_context *ctx, u32 *addrs)
> -{
> -	const struct bpf_insn *insn = fp->insnsi;
> -	bool func_addr_fixed;
> -	u64 func_addr;
> -	u32 tmp_idx;
> -	int i, j, ret;
> -
> -	for (i = 0; i < fp->len; i++) {
> -		/*
> -		 * During the extra pass, only the branch target addresses for
> -		 * the subprog calls need to be fixed. All other instructions
> -		 * can left untouched.
> -		 *
> -		 * The JITed image length does not change because we already
> -		 * ensure that the JITed instruction sequence for these calls
> -		 * are of fixed length by padding them with NOPs.
> -		 */
> -		if (insn[i].code == (BPF_JMP | BPF_CALL) &&
> -		    insn[i].src_reg == BPF_PSEUDO_CALL) {
> -			ret = bpf_jit_get_func_addr(fp, &insn[i], true,
> -						    &func_addr,
> -						    &func_addr_fixed);

I don't see you updating calls to bpf_jit_get_func_addr() in 
bpf_jit_build_body() to set extra_pass to true. Afaics, that's required 
to get the correct address to be branched to for subprogs.


- Naveen