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

[Christophe Leroy <christophe.leroy@csgroup.eu>]

Le 26/09/2023 à 00:50, Song Liu a écrit :
> 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.

If I remember correctry, patch 1 introduces a huge performance 
degradation, which gets then improved with this patch.

As I said before, I'd expect patch 4 to go first then get 
bpf_arch_text_copy() be implemented with patch_instructions() directly.

Christophe

> 
> [...]
> 
>> @@ -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;
>>   }
>>