Re: [PATCH 08/13] hw/xtensa/xtfpga: Replace memcpy()+tswap32() by stl_endian_p()

[Thomas Huth <thuth@redhat.com>]

On 03/10/2024 23.40, Philippe Mathieu-Daudé wrote:
> On 3/10/24 23:34, Pierrick Bouvier wrote:
>> On 10/3/24 14:31, Pierrick Bouvier wrote:
>>> On 10/3/24 13:48, Philippe Mathieu-Daudé wrote:
>>>> On 3/10/24 18:04, Pierrick Bouvier wrote:
>>>>> On 10/3/24 09:02, Philippe Mathieu-Daudé wrote:
>>>>>> On 30/9/24 16:32, Thomas Huth wrote:
>>>>>>> On 30/09/2024 09.34, Philippe Mathieu-Daudé wrote:
>>>>>>>> Replace a pair of memcpy() + tswap32() by stl_endian_p(),
>>>>>>>> which also swap the value using target endianness.
>>>>>>>>
>>>>>>>> Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
>>>>>>>> ---
>>>>>>>>      hw/xtensa/xtfpga.c | 6 ++----
>>>>>>>>      1 file changed, 2 insertions(+), 4 deletions(-)
>>>>>>>>
>>>>>>>> diff --git a/hw/xtensa/xtfpga.c b/hw/xtensa/xtfpga.c
>>>>>>>> index 228f00b045..521fe84b01 100644
>>>>>>>> --- a/hw/xtensa/xtfpga.c
>>>>>>>> +++ b/hw/xtensa/xtfpga.c
>>>>>>>> @@ -438,11 +438,9 @@ static void xtfpga_init(const XtfpgaBoardDesc
>>>>>>>> *board, MachineState *machine)
>>>>>>>>                  const size_t boot_sz = TARGET_BIG_ENDIAN ?
>>>>>>>> sizeof(boot_be)
>>>>>>>>                                                           :
>>>>>>>> sizeof(boot_le);
>>>>>>>>                  uint8_t *boot = TARGET_BIG_ENDIAN ? boot_be : boot_le;
>>>>>>>> -            uint32_t entry_pc = tswap32(entry_point);
>>>>>>>> -            uint32_t entry_a2 = tswap32(tagptr);
>>>>>>>> -            memcpy(boot + 4, &entry_pc, sizeof(entry_pc));
>>>>>>>> -            memcpy(boot + 8, &entry_a2, sizeof(entry_a2));
>>>>>>>> +            stl_endian_p(TARGET_BIG_ENDIAN, boot + 4, entry_point);
>>>>>>>> +            stl_endian_p(TARGET_BIG_ENDIAN, boot + 8, tagptr);
>>>>>>>
>>>>>>> Why don't you simply use stl_p() here?
>>>>>>
>>>>>> We want to remove the tswap32() calls...
>>>>>>
>>>>>
>>>>> I think is point is that you could directly use stl_be_p, instead of
>>>>> stl_endian_p(TARGET_BIT_ENDIAN, ...).
>>>>
>>>> TARGET_BIG_ENDIAN is defined as 0 on little endian, and 1 on big one.
>>>>
>>>> The following change isn't worth it:
>>>>
>>>>      if (TARGET_BIG_ENDIAN) {
>>>>        stl_be_p(boot + 8, tagptr);
>>>>      } else {
>>>>        stl_le_p(boot + 8, tagptr);
>>>>      }
>>>>
>>>> Maybe I'm missing Thomas point, as the xtfpga machines are available
>>>> for both xtensa-softmmu (LE) and xtensaeb-softmmu (BE).
>>>>
>>>>> I don't know if your intent is to make be/le variant "private" and
>>>>> relies only on endian_p though.
>>>>
>>>> My intent is to enforce endian agnostic API uses when possible, and
>>>> use LE/BE specific variant when it is known at build time.
>>>
>>> Oh ok, it's me who missed your point then.
>>> For some reason, I thought we were always calling big endian variant.
>>>
>>> Thus, your implementation makes totally sense.
>>>
>>> Let's see if Thomas meant something different.
>>> Else,
>>> Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
>>
>> Looking more closely,
>> stl_p is already correctly defined when you know at compile time your 
>> target endianness. So Thomas was referring to this.
>>
>> https://gitlab.com/qemu-project/qemu/-/blame/master/include/exec/cpu- 
>> all.h?ref_type=heads#L49
> 
> OK I guess I'm seeing Thomas point now; this series cover was not clear
> enough. The goal is to remove TARGET_BIG_ENDIAN so we can build half
> objects and do a little step toward the single binary.

Ok, that piece of information was missing in the patch description, indeed.
But the tswap function should already work with common code, see the 
target_needs_bswap() stuff in include/exec/tswap.h, so maybe this change 
here even is not needed at all?

  Thomas