Re: [PATCH v4 2/5] powerpc/bpf: Move out dummy_tramp_addr after Long branch stub

[Hari Bathini <hbathini@linux.ibm.com>]

On 18/05/26 3:10 am, adubey@linux.ibm.com wrote:
> From: Abhishek Dubey <adubey@linux.ibm.com>
> 
> Move the long branch address space to the bottom of the long
> branch stub. This allows uninterrupted disassembly until the
> last 8 bytes. Exclude these last bytes from the overall
> program length to prevent failure in assembly generation.
> Also, align dummy_tramp_addr field with 8-byte boundary.
> 
> Following is disassembler output for test program with moved down
> dummy_tramp_addr field:
> .....
> .....
> pc:68    left:44     a6 03 08 7c  :  mtlr 0
> pc:72    left:40     bc ff ff 4b  :  b .-68
> pc:76    left:36     a6 02 68 7d  :  mflr 11
> pc:80    left:32     05 00 9f 42  :  bcl 20, 31, .+4
> pc:84    left:28     a6 02 88 7d  :  mflr 12
> pc:88    left:24     14 00 8c e9  :  ld 12, 20(12)
> pc:92    left:20     a6 03 89 7d  :  mtctr 12
> pc:96    left:16     a6 03 68 7d  :  mtlr 11
> pc:100   left:12     20 04 80 4e  :  bctr
> pc:104   left:8      c0 34 1d 00  :
> 
> Failure log:
> Can't disasm instruction at offset 104: c0 34 1d 00 00 00 00 c0
> Disassembly logic can truncate at 104, ignoring last 8 bytes.
> 
> Update the dummy_tramp_addr field offset calculation from the end
> of the program to reflect its new location, for bpf_arch_text_poke()
> to update the actual trampoline's address in this field.
> 
> All BPF trampoline selftests continue to pass with this patch applied.
> 
> Signed-off-by: Abhishek Dubey <adubey@linux.ibm.com>
> ---
>   arch/powerpc/net/bpf_jit_comp.c | 34 +++++++++++++++++++--------------
>   1 file changed, 20 insertions(+), 14 deletions(-)
> 
> diff --git a/arch/powerpc/net/bpf_jit_comp.c b/arch/powerpc/net/bpf_jit_comp.c
> index ef7614177cb1..b73bc9295c31 100644
> --- a/arch/powerpc/net/bpf_jit_comp.c
> +++ b/arch/powerpc/net/bpf_jit_comp.c
> @@ -57,19 +57,21 @@ void bpf_jit_build_fentry_stubs(u32 *image, u32 *fimage, struct codegen_context
>   	 * In the final pass, align the mis-aligned dummy_tramp_addr field
>   	 * in the fimage. The alignment NOP must appear before OOL stub,
>   	 * to make ool_stub_idx & long_branch_stub_idx constant from end.
> +	 *
> +	 * The dummy_tramp_addr field is placed at bottom of Long branch stub.
>   	 */
>   #ifdef CONFIG_PPC64
>   	if (fimage && image) {
>   		/*
>   		 * pc points to first instruction of OOL stub,
> -		 * dummy_tramp_addr is past 4/3 instructions depending on
> +		 * dummy_tramp_addr is past 11/10 instructions depending on
>   		 * CONFIG_PPC_FTRACE_OUT_OF_LINE is enabled/not respectively.
>   		 *
>   		 * The decision to emit alignment NOP must depend on the alignment
>   		 * of dummy_tramp_addr field.
>   		 */
>   		unsigned long pc = (unsigned long)fimage + CTX_NIA(ctx);

> -		pc += IS_ENABLED(CONFIG_PPC_FTRACE_OUT_OF_LINE) ? 4 : 3;
> +		pc += IS_ENABLED(CONFIG_PPC_FTRACE_OUT_OF_LINE) ? 11 : 10;

To get the address, should multiply the instruction count with 4..

     pc += (IS_ENABLED(CONFIG_PPC_FTRACE_OUT_OF_LINE) ? 11 : 10) * 4;

Also, pc may not be appropriate name here. We are essentially
calculating the pointer address of dummy_tramp_addr. `addrp` maybe?

- Hari