Re: [bpf-next 3/6] bpf,powerpc: Implement bpf_addr_space_cast instruction

[Saket Kumar Bhaskar <skb99@linux.ibm.com>]

On Tue, Aug 05, 2025 at 09:29:07AM +0200, Christophe Leroy wrote:
> 
> 
> 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@linux.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.
> 
Alright Chris, will define and implement it here.
> > +				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));
>