Re: [PATCH v6 1/4] bswap: Add the ability to store to an unaligned 24 bit field

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

On 19/5/23 18:24, Jonathan Cameron wrote:
> On Fri, 19 May 2023 18:08:30 +0200
> Philippe Mathieu-Daudé <philmd@linaro.org> wrote:
> 
>> On 19/5/23 16:18, Jonathan Cameron wrote:
>>> From: Ira Weiny <ira.weiny@intel.com>
>>>
>>> CXL has 24 bit unaligned fields which need to be stored to.  CXL is
>>> specified as little endian.
>>>
>>> Define st24_le_p() and the supporting functions to store such a field
>>> from a 32 bit host native value.
>>>
>>> The use of b, w, l, q as the size specifier is limiting.  So "24" was
>>> used for the size part of the function name.
>>>
>>> Signed-off-by: Ira Weiny <ira.weiny@intel.com>
>>> Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
>>> ---
>>>    docs/devel/loads-stores.rst |  1 +
>>>    include/qemu/bswap.h        | 27 +++++++++++++++++++++++++++
>>>    2 files changed, 28 insertions(+)
>>>
>>> diff --git a/docs/devel/loads-stores.rst b/docs/devel/loads-stores.rst
>>> index d2cefc77a2..82a79e91d9 100644
>>> --- a/docs/devel/loads-stores.rst
>>> +++ b/docs/devel/loads-stores.rst
>>> @@ -36,6 +36,7 @@ store: ``st{size}_{endian}_p(ptr, val)``
>>>    ``size``
>>>     - ``b`` : 8 bits
>>>     - ``w`` : 16 bits
>>> + - ``24`` : 24 bits
>>>     - ``l`` : 32 bits
>>>     - ``q`` : 64 bits
>>>    
>>> diff --git a/include/qemu/bswap.h b/include/qemu/bswap.h
>>> index 15a78c0db5..f546b1fc06 100644
>>> --- a/include/qemu/bswap.h
>>> +++ b/include/qemu/bswap.h
>>> @@ -8,11 +8,25 @@
>>>    #undef  bswap64
>>>    #define bswap64(_x) __builtin_bswap64(_x)
>>>    
>>> +static inline uint32_t bswap24(uint32_t x)
>>> +{
>>> +    assert((x & 0xff000000U) == 0);
>>
>> Asserting here is a bit violent. In particular because there is no
>> contract description that x should be less than N bits for bswapN()
>> in "qemu/bswap.h" API.
>>
>> But if you rather to assert ...
> 
> 
> I'm fine either way.  You asked for it when reviewing v4...
> https://lore.kernel.org/all/28a9d97a-b252-a33f-1ac0-cd36264b29ab@linaro.org/

Never too late to improve oneself ;)

One month later I'm afraid the assertion would fire too often,
resulting in unnecessary pain to developers (in particular the
non-QEMU ones).

>>
>>> +
>>> +    return (((x & 0x000000ffU) << 16) |
>>> +            ((x & 0x0000ff00U) <<  0) |
>>> +            ((x & 0x00ff0000U) >> 16));
>>> +}
>>> +
>>>    static inline void bswap16s(uint16_t *s)
>>>    {
>>>        *s = __builtin_bswap16(*s);
>>>    }
>>>    
>>> +static inline void bswap24s(uint32_t *s)
>>> +{
>>> +    *s = bswap24(*s & 0x00ffffffU);
>>
>> ... and sanitize the value here ...
>>
>>> +}
>>> +
>>>    static inline void bswap32s(uint32_t *s)
>>>    {
>>>        *s = __builtin_bswap32(*s);
>>> @@ -26,11 +40,13 @@ static inline void bswap64s(uint64_t *s)
>>>    #if HOST_BIG_ENDIAN
>>>    #define be_bswap(v, size) (v)
>>>    #define le_bswap(v, size) glue(__builtin_bswap, size)(v)
>>> +#define le_bswap24(v) bswap24(v)
>>
>> ... then shouldn't you sanitize also here?
>>
> 
> Good point. I forgot that detail whilst fighting with s390
> cross builds earlier ;)
> 
>> Personally I'd just drop the assertion.
> 
> I'm fine with doing that.
> 
> Jonathan
> 
>>
>>>    #define be_bswaps(v, size)
>>>    #define le_bswaps(p, size) \
>>>                do { *p = glue(__builtin_bswap, size)(*p); } while (0)
>>>    #else
>>>    #define le_bswap(v, size) (v)
>>> +#define le_bswap24(v) (v)
>>>    #define be_bswap(v, size) glue(__builtin_bswap, size)(v)
>>>    #define le_bswaps(v, size)
>>>    #define be_bswaps(p, size) \
>>> @@ -176,6 +192,7 @@ CPU_CONVERT(le, 64, uint64_t)
>>>     * size is:
>>>     *   b: 8 bits
>>>     *   w: 16 bits
>>> + *   24: 24 bits
>>>     *   l: 32 bits
>>>     *   q: 64 bits
>>>     *
>>> @@ -248,6 +265,11 @@ static inline void stw_he_p(void *ptr, uint16_t v)
>>>        __builtin_memcpy(ptr, &v, sizeof(v));
>>>    }
>>>    
>>> +static inline void st24_he_p(void *ptr, uint32_t v)
>>> +{
>>> +    __builtin_memcpy(ptr, &v, 3);
>>> +}
>>> +
>>>    static inline int ldl_he_p(const void *ptr)
>>>    {
>>>        int32_t r;
>>> @@ -297,6 +319,11 @@ static inline void stw_le_p(void *ptr, uint16_t v)
>>>        stw_he_p(ptr, le_bswap(v, 16));
>>>    }
>>>    
>>> +static inline void st24_le_p(void *ptr, uint32_t v)
>>> +{
>>> +    st24_he_p(ptr, le_bswap24(v));
>>> +}
>>> +
>>>    static inline void stl_le_p(void *ptr, uint32_t v)
>>>    {
>>>        stl_he_p(ptr, le_bswap(v, 32));
>>
>> Conditional to removing the assertion in bswap24():
>> Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
>>
>