bswap: Add the ability to store to an unaligned 24 bit field

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.

Reviewed-by: Fan Ni <fan.ni@samsung.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Message-Id: <20230526170010.574-2-Jonathan.Cameron@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

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
 
@@ -65,6 +66,7 @@ of size ``sz`` bytes.
 Regexes for git grep
  - ``\<ld[us]\?[bwlq]\(_[hbl]e\)\?_p\>``
  - ``\<st[bwlq]\(_[hbl]e\)\?_p\>``
+ - ``\<st24\(_[hbl]e\)\?_p\>``
  - ``\<ldn_\([hbl]e\)?_p\>``
  - ``\<stn_\([hbl]e\)?_p\>``
 

include/qemu/bswap.h

@@ -8,11 +8,23 @@
 #undef  bswap64
 #define bswap64(_x) __builtin_bswap64(_x)
 
+static inline uint32_t bswap24(uint32_t x)
+{
+    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);
+}
+
 static inline void bswap32s(uint32_t *s)
 {
     *s = __builtin_bswap32(*s);
@@ -26,11 +38,13 @@ 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)
 #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 +190,7 @@ 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 +263,11 @@ 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 +317,11 @@ 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));