Re: [PATCH v4 4/5] powerpc/code-patching: introduce patch_instructions()

[Song Liu <song@kernel.org>]

On Fri, Sep 8, 2023 at 6:28 AM Hari Bathini <hbathini@linux.ibm.com> wrote:
>
> patch_instruction() entails setting up pte, patching the instruction,
> clearing the pte and flushing the tlb. If multiple instructions need
> to be patched, every instruction would have to go through the above
> drill unnecessarily. Instead, introduce function patch_instructions()
> that sets up the pte, clears the pte and flushes the tlb only once per
> page range of instructions to be patched. This adds a slight overhead
> to patch_instruction() call while improving the patching time for
> scenarios where more than one instruction needs to be patched.
>
> Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>

I didn't see this one when I reviewed 1/5. Please ignore that comment.

[...]

> @@ -307,11 +312,22 @@ static int __do_patch_instruction_mm(u32 *addr, ppc_inst_t instr)
>
>         orig_mm = start_using_temp_mm(patching_mm);
>
> -       err = __patch_instruction(addr, instr, patch_addr);
> +       while (len > 0) {
> +               instr = ppc_inst_read(code);
> +               ilen = ppc_inst_len(instr);
> +               err = __patch_instruction(addr, instr, patch_addr);

It appears we are still repeating a lot of work here. For example, with
fill_insn == true, we don't need to repeat ppc_inst_read().

Can we do this with a memcpy or memset like functions?

> +               /* hwsync performed by __patch_instruction (sync) if successful */
> +               if (err) {
> +                       mb();  /* sync */
> +                       break;
> +               }
>
> -       /* hwsync performed by __patch_instruction (sync) if successful */
> -       if (err)
> -               mb();  /* sync */
> +               len -= ilen;
> +               patch_addr = patch_addr + ilen;
> +               addr = (void *)addr + ilen;
> +               if (!fill_insn)
> +                       code = code + ilen;

It took me a while to figure out what "fill_insn" means. Maybe call it
"repeat_input" or something?

Thanks,
Song

> +       }
>
>         /* context synchronisation performed by __patch_instruction (isync or exception) */
>         stop_using_temp_mm(patching_mm, orig_mm);
> @@ -328,16 +344,21 @@ static int __do_patch_instruction_mm(u32 *addr, ppc_inst_t instr)
>         return err;
>  }
>