Re: [PATCH 7/7] tcg/ppc: Use prefixed instructions for tcg_out_goto_tb

[Richard Henderson <richard.henderson@linaro.org>]

On 8/6/23 05:55, Nicholas Piggin wrote:
> On Sat Aug 5, 2023 at 7:33 AM AEST, Richard Henderson wrote:
>> When a direct branch is out of range, we can load the destination for
>> the indirect branch using PLA (for 16GB worth of buffer) and PLD from
>> the TranslationBlock for everything larger.
>>
>> This means the patch affects exactly one instruction: B (plus filler),
>> PLA or PLD.  Which means we can update and execute the patch atomically.
>>
>> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
>> ---
>>   tcg/ppc/tcg-target.c.inc | 76 ++++++++++++++++++++++++++++++----------
>>   1 file changed, 58 insertions(+), 18 deletions(-)
>>
>> diff --git a/tcg/ppc/tcg-target.c.inc b/tcg/ppc/tcg-target.c.inc
>> index 5b243b2353..47c71bb5f2 100644
>> --- a/tcg/ppc/tcg-target.c.inc
>> +++ b/tcg/ppc/tcg-target.c.inc
>> @@ -2642,31 +2642,41 @@ static void tcg_out_goto_tb(TCGContext *s, int which)
>>       uintptr_t ptr = get_jmp_target_addr(s, which);
>>   
>>       if (USE_REG_TB) {
>> +        /*
>> +         * With REG_TB, we must always use indirect branching,
>> +         * so that the branch destination and TCG_REG_TB match.
>> +         */
>>           ptrdiff_t offset = tcg_tbrel_diff(s, (void *)ptr);
>>           tcg_out_mem_long(s, LD, LDX, TCG_REG_TB, TCG_REG_TB, offset);
>> -
>> -        /* TODO: Use direct branches when possible. */
>> -        set_jmp_insn_offset(s, which);
>>           tcg_out32(s, MTSPR | RS(TCG_REG_TB) | CTR);
>> -
>>           tcg_out32(s, BCCTR | BO_ALWAYS);
>>   
>>           /* For the unlinked case, need to reset TCG_REG_TB.  */
>>           set_jmp_reset_offset(s, which);
>>           tcg_out_mem_long(s, ADDI, ADD, TCG_REG_TB, TCG_REG_TB,
>>                            -tcg_current_code_size(s));
>> +        return;
>> +    }
>> +
>> +    if (have_isa_3_10) {
>> +        /* Align, so that we can patch 8 bytes atomically. */
>> +        if ((uintptr_t)s->code_ptr & 7) {
>> +            tcg_out32(s, NOP);
>> +        }
>> +        set_jmp_insn_offset(s, which);
>> +        /* Direct branch will be patched by tb_target_set_jmp_target. */
>> +        tcg_out_mls_d(s, ADDI, TCG_REG_TMP1, 0, 0, 1);
>>       } else {
>>           /* Direct branch will be patched by tb_target_set_jmp_target. */
>> -        set_jmp_insn_offset(s, which);
>> -        tcg_out32(s, NOP);
>> -
>> +        tcg_out32(s, B);
>>           /* When branch is out of range, fall through to indirect. */
>>           tcg_out_movi(s, TCG_TYPE_PTR, TCG_REG_TMP1, ptr - (int16_t)ptr);
>>           tcg_out_ld(s, TCG_TYPE_PTR, TCG_REG_TMP1, TCG_REG_TMP1, (int16_t)ptr);
>> -        tcg_out32(s, MTSPR | RS(TCG_REG_TMP1) | CTR);
>> -        tcg_out32(s, BCCTR | BO_ALWAYS);
>> -        set_jmp_reset_offset(s, which);
>>       }
>> +
>> +    tcg_out32(s, MTSPR | RS(TCG_REG_TMP1) | CTR);
>> +    tcg_out32(s, BCCTR | BO_ALWAYS);
>> +    set_jmp_reset_offset(s, which);
>>   }
>>   
>>   void tb_target_set_jmp_target(const TranslationBlock *tb, int n,
>> @@ -2674,20 +2684,50 @@ void tb_target_set_jmp_target(const TranslationBlock *tb, int n,
>>   {
>>       uintptr_t addr = tb->jmp_target_addr[n];
>>       intptr_t diff = addr - jmp_rx;
>> -    tcg_insn_unit insn;
>>   
>>       if (USE_REG_TB) {
>>           return;
>>       }
>>   
>> -    if (in_range_b(diff)) {
>> -        insn = B | (diff & 0x3fffffc);
>> -    } else {
>> -        insn = NOP;
>> -    }
>> +    if (have_isa_3_10) {
>> +        tcg_insn_unit insn1, insn2;
>> +        uint64_t pair;
>>   
>> -    qatomic_set((uint32_t *)jmp_rw, insn);
>> -    flush_idcache_range(jmp_rx, jmp_rw, 4);
>> +        if (in_range_b(diff)) {
>> +            insn1 = B | (diff & 0x3fffffc);
>> +            insn2 = NOP;
>> +        } else if (diff == sextract64(diff, 0, 34)) {
>> +            /* PLA tmp1, diff */
>> +            insn1 = OPCD(1) | (2 << 24) | (1 << 20) | ((diff >> 16) & 0x3ffff);
>> +            insn2 = ADDI | TAI(TCG_REG_TMP1, 0, diff);
>> +        } else {
>> +            addr = (uintptr_t)&tb->jmp_target_addr[n];
>> +            diff = addr - jmp_rx;
>> +            tcg_debug_assert(diff == sextract64(diff, 0, 34));
>> +            /* PLD tmp1, diff */
>> +            insn1 = OPCD(1) | (1 << 20) | ((diff >> 16) & 0x3ffff);
>> +            insn2 = PLD | TAI(TCG_REG_TMP1, 0, diff);
>> +        }
> 
> B is a "patch class" word instruction as per CMODX in the ISA, which may
> be patched to/from other instructions without a flush+isync sequence
> betwen. So that part is okay, at least if you were just patching the B
> word. But patching between the PLA and PLD I don't think is kosher per
> ISA.
> 
> I struggle a bit with this part of the ISA, particularly with prefix
> instructions (it only talks about patching 4 bytes at a time).
> 
> If we patch something it has to go through a patch instruction, which
> is a direct branch, trap, or nop. I think that makes this non-trivial.
> 
> It could work if you only patched between B and PLD. B->PLD would have
> to patch the suffix word first, possibly with an interleaving sync, and
> then the prefix. PLD->B could just patch the B word.
> 
> How much would losing the PLA hurt?

Really?  I can't imagine how some icache would see a torn prefixed insn given an atomic 
store (CMODX talks about prefixed instructions which "may be unaligned" -- but what if 
they are not?).

But if patching an aligned prefixed insn isn't allowed, I would patch between B and NOP, 
leave the PLD alone on the fall-through path, and drop the PLA.


r~