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

[Jonathan Cameron via <qemu-devel@nongnu.org>]

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/


> 
> > +
> > +    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>
>