[RFC PATCH 14/25] qemu/bswap: Introduce load/store for aligned pointer

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

When the pointer alignment is known to be safe, we can
directly swap the data in place, without having to rely
on the compiler builtin code.

Load/store methods expecting aligned pointer use the 'a'
infix. For example to read a 16-bit unsigned value stored
in little endianess at an unaligned pointer:

  val = lduw_le_p(&unaligned_ptr);

then to store it in big endianess at an aligned pointer:

  stw_be_ap(&aligned_ptr, val);

Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
 docs/devel/loads-stores.rst | 27 ++++++++++++++++-----------
 include/qemu/bswap.h        | 22 ++++++++++++++++++++++
 2 files changed, 38 insertions(+), 11 deletions(-)

diff --git a/docs/devel/loads-stores.rst b/docs/devel/loads-stores.rst
index 568274baec0..88493ba1293 100644
--- a/docs/devel/loads-stores.rst
+++ b/docs/devel/loads-stores.rst
@@ -13,20 +13,21 @@ documentation of each API -- for that you should look at the
 documentation comments in the relevant header files.
 
 
-``ld*_p and st*_p``
-~~~~~~~~~~~~~~~~~~~
+``ld*_[a]p and st*_[a]p``
+~~~~~~~~~~~~~~~~~~~~~~~~~
 
 These functions operate on a host pointer, and should be used
 when you already have a pointer into host memory (corresponding
 to guest ram or a local buffer). They deal with doing accesses
 with the desired endianness and with correctly handling
-potentially unaligned pointer values.
+potentially unaligned pointer values. If the pointer alignment
+is known to be safe, then the aligned functions can be used.
 
 Function names follow the pattern:
 
-load: ``ld{sign}{size}_{endian}_p(ptr)``
+load: ``ld{sign}{size}_{endian}_{aligned}p(ptr)``
 
-store: ``st{size}_{endian}_p(ptr, val)``
+store: ``st{size}_{endian}_{aligned}p(ptr, val)``
 
 ``sign``
  - (empty) : for 32 or 64 bit sizes
@@ -49,24 +50,28 @@ The ``_{endian}`` infix is omitted for target-endian accesses.
 The target endian accessors are only available to source
 files which are built per-target.
 
+By using the ``_{aligned}`` infix, unsafe optimizations might be used,
+however unaligned pointer might trigger an exception and abort the
+process.
+
 There are also functions which take the size as an argument:
 
-load: ``ldn{endian}_p(ptr, sz)``
+load: ``ldn{endian}_{aligned}p(ptr, sz)``
 
 which performs an unsigned load of ``sz`` bytes from ``ptr``
 as an ``{endian}`` order value and returns it in a uint64_t.
 
-store: ``stn{endian}_p(ptr, sz, val)``
+store: ``stn{endian}_{aligned}p(ptr, sz, val)``
 
 which stores ``val`` to ``ptr`` as an ``{endian}`` order value
 of size ``sz`` bytes.
 
 
 Regexes for git grep
- - ``\<ld[us]\?[bwlq]\(_[hbl]e\)\?_p\>``
- - ``\<st[bwlq]\(_[hbl]e\)\?_p\>``
- - ``\<ldn_\([hbl]e\)?_p\>``
- - ``\<stn_\([hbl]e\)?_p\>``
+ - ``\<ld[us]\?[bwlq]\(_[hbl]e\)\?_a?p\>``
+ - ``\<st[bwlq]\(_[hbl]e\)\?_a?p\>``
+ - ``\<ldn_\([hbl]e\)?_a?p\>``
+ - ``\<stn_\([hbl]e\)?_a?p\>``
 
 ``cpu_{ld,st}*_mmuidx_ra``
 ~~~~~~~~~~~~~~~~~~~~~~~~~~
diff --git a/include/qemu/bswap.h b/include/qemu/bswap.h
index 4cd120ca014..3f272c3cb46 100644
--- a/include/qemu/bswap.h
+++ b/include/qemu/bswap.h
@@ -350,25 +350,47 @@ static inline void st ## size ## _he_p(void *ptr, vtype v)\
     __builtin_memcpy(ptr, &v, sizeof(v));\
 }
 
+#define LD_CONVERT_ALIGNED(bits, rtype, vtype, size)\
+static inline rtype ld ## size ## _he_ap(const void *ptr)\
+{\
+    return *(vtype *)ptr;\
+}
+
+#define ST_CONVERT_ALIGNED(bits, vtype, size)\
+static inline void st ## size ## _he_ap(void *ptr, vtype v)\
+{\
+    *(vtype *)ptr = v;\
+}
+
 #define LD_CONVERT_END(endian, bits, rtype, vtype, size)\
 static inline rtype ld ## size ## _ ## endian ## _p(const void *ptr)\
 {\
     return (vtype)glue(endian, _bswap)(ld ## size ## _he_p(ptr), bits);\
+}\
+static inline rtype ld ## size ## _ ## endian ## _ap(const void *ptr)\
+{\
+    return (vtype)glue(endian, _bswap)(ld ## size ## _he_ap(ptr), bits);\
 }
 
 #define ST_CONVERT_END(endian, bits, vtype, size)\
 static inline void st ## size ## _ ## endian ## _p(void *ptr, vtype v)\
 {\
     st ## size ## _he_p(ptr, glue(endian, _bswap)(v, bits));\
+}\
+static inline void st ## size ## _ ## endian ## _ap(void *ptr, vtype v)\
+{\
+    st ## size ## _he_ap(ptr, glue(endian, _bswap)(v, bits));\
 }
 
 #define LD_CONVERT(bits, rtype, vtype, size)\
     LD_CONVERT_UNALIGNED(bits, rtype, vtype, size)\
+    LD_CONVERT_ALIGNED(bits, rtype, vtype, size)\
     LD_CONVERT_END(le, bits, rtype, vtype, size)\
     LD_CONVERT_END(be, bits, rtype, vtype, size)
 
 #define ST_CONVERT(bits, vtype, size)\
     ST_CONVERT_UNALIGNED(bits, vtype, size)\
+    ST_CONVERT_ALIGNED(bits, vtype, size)\
     ST_CONVERT_END(le, bits, vtype, size)\
     ST_CONVERT_END(be, bits, vtype, size)
 
-- 
2.26.3