linux-kernel - Re: [bpf-next 3/6] bpf,powerpc: Implement bpf_addr_space

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Message-ID: <bc6ef2d1-da06-4b29-a0ec-67549074bc0b@csgroup.eu>
Date: Tue, 5 Aug 2025 09:29:07 +0200
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Saket Kumar Bhaskar <skb99@...ux.ibm.com>, bpf@...r.kernel.org,
 linuxppc-dev@...ts.ozlabs.org, linux-kselftest@...r.kernel.org,
 linux-kernel@...r.kernel.org
Cc: hbathini@...ux.ibm.com, sachinpb@...ux.ibm.com, venkat88@...ux.ibm.com,
 andrii@...nel.org, eddyz87@...il.com, mykolal@...com, ast@...nel.org,
 daniel@...earbox.net, martin.lau@...ux.dev, song@...nel.org,
 yonghong.song@...ux.dev, john.fastabend@...il.com, kpsingh@...nel.org,
 sdf@...ichev.me, haoluo@...gle.com, jolsa@...nel.org, naveen@...nel.org,
 maddy@...ux.ibm.com, mpe@...erman.id.au, npiggin@...il.com,
 memxor@...il.com, iii@...ux.ibm.com, shuah@...nel.org
Subject: Re: [bpf-next 3/6] bpf,powerpc: Implement bpf_addr_space_cast
 instruction



Le 05/08/2025 à 08:27, Saket Kumar Bhaskar a écrit :
> LLVM generates bpf_addr_space_cast instruction while translating
> pointers between native (zero) address space and
> __attribute__((address_space(N))). The addr_space=0 is reserved as
> bpf_arena address space.
> 
> rY = addr_space_cast(rX, 0, 1) is processed by the verifier and
> converted to normal 32-bit move: wX = wY.
> 
> rY = addr_space_cast(rX, 1, 0) : used to convert a bpf arena pointer to
> a pointer in the userspace vma. This has to be converted by the JIT.
> 
> Signed-off-by: Saket Kumar Bhaskar <skb99@...ux.ibm.com>
> ---
>   arch/powerpc/net/bpf_jit.h        |  1 +
>   arch/powerpc/net/bpf_jit_comp.c   |  6 ++++++
>   arch/powerpc/net/bpf_jit_comp64.c | 11 +++++++++++
>   3 files changed, 18 insertions(+)
> 
> diff --git a/arch/powerpc/net/bpf_jit.h b/arch/powerpc/net/bpf_jit.h
> index 2d095a873305..748e30e8b5b4 100644
> --- a/arch/powerpc/net/bpf_jit.h
> +++ b/arch/powerpc/net/bpf_jit.h
> @@ -165,6 +165,7 @@ struct codegen_context {
>   	unsigned int exentry_idx;
>   	unsigned int alt_exit_addr;
>   	u64 arena_vm_start;
> +	u64 user_vm_start;
>   };
>   
>   #define bpf_to_ppc(r)	(ctx->b2p[r])
> diff --git a/arch/powerpc/net/bpf_jit_comp.c b/arch/powerpc/net/bpf_jit_comp.c
> index 35bfdf4d8785..2b3f90930c27 100644
> --- a/arch/powerpc/net/bpf_jit_comp.c
> +++ b/arch/powerpc/net/bpf_jit_comp.c
> @@ -205,6 +205,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *fp)
>   	/* Make sure that the stack is quadword aligned. */
>   	cgctx.stack_size = round_up(fp->aux->stack_depth, 16);
>   	cgctx.arena_vm_start = bpf_arena_get_kern_vm_start(fp->aux->arena);
> +	cgctx.user_vm_start = bpf_arena_get_user_vm_start(fp->aux->arena);
>   
>   	/* Scouting faux-generate pass 0 */
>   	if (bpf_jit_build_body(fp, NULL, NULL, &cgctx, addrs, 0, false)) {
> @@ -441,6 +442,11 @@ bool bpf_jit_supports_kfunc_call(void)
>   	return true;
>   }
>   
> +bool bpf_jit_supports_arena(void)
> +{
> +	return IS_ENABLED(CONFIG_PPC64);
> +}
> +
>   bool bpf_jit_supports_far_kfunc_call(void)
>   {
>   	return IS_ENABLED(CONFIG_PPC64);
> diff --git a/arch/powerpc/net/bpf_jit_comp64.c b/arch/powerpc/net/bpf_jit_comp64.c
> index 16e62766c757..d4fe4dacf2d6 100644
> --- a/arch/powerpc/net/bpf_jit_comp64.c
> +++ b/arch/powerpc/net/bpf_jit_comp64.c
> @@ -812,6 +812,17 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, u32 *fimage, struct code
>   		 */
>   		case BPF_ALU | BPF_MOV | BPF_X: /* (u32) dst = src */
>   		case BPF_ALU64 | BPF_MOV | BPF_X: /* dst = src */
> +
> +			if (insn_is_cast_user(&insn[i])) {
> +				EMIT(PPC_RAW_RLDICL(tmp1_reg, src_reg, 0, 32));

Define and use PPC_RAW_RLDICL_DOT to avoid the CMPDI below.

> +				PPC_LI64(dst_reg, (ctx->user_vm_start & 0xffffffff00000000UL));
> +				EMIT(PPC_RAW_CMPDI(tmp1_reg, 0));
> +				PPC_BCC_SHORT(COND_EQ, (ctx->idx + 2) * 4);
> +				EMIT(PPC_RAW_OR(tmp1_reg, dst_reg, tmp1_reg));
> +				EMIT(PPC_RAW_MR(dst_reg, tmp1_reg));
> +				break;
> +			}
> +
>   			if (imm == 1) {
>   				/* special mov32 for zext */
>   				EMIT(PPC_RAW_RLWINM(dst_reg, dst_reg, 0, 0, 31));