[PATCH for-8.0 02/29] include/exec/memop: Add bits describing atomicity

From: Richard Henderson <richard.henderson@linaro.org>
To: qemu-devel@nongnu.org
Subject: [PATCH for-8.0 02/29] include/exec/memop: Add bits describing atomicity
Date: Fri, 18 Nov 2022 01:47:27 -0800	[thread overview]
Message-ID: <20221118094754.242910-3-richard.henderson@linaro.org> (raw)
In-Reply-To: <20221118094754.242910-1-richard.henderson@linaro.org>

These bits may be used to describe the precise atomicity
requirements of the guest, which may then be used to
constrain the methods by which it may be emulated by the host.

For instance, the AArch64 LDP (32-bit) instruction changes
semantics with ARMv8.4 LSE2, from

  MO_64 | MO_ATMAX_4 | MO_ATOM_IFALIGN
  (64-bits, single-copy atomic only on 4 byte units,
   nonatomic if not aligned by 4),

to

  MO_64 | MO_ATMAX_SIZE | MO_ATOM_WITHIN16
  (64-bits, single-copy atomic within a 16 byte block)

The former may be implemented with two 4 byte loads, or
a single 8 byte load if that happens to be efficient on
the host.  The latter may not, and may also require a
helper when misaligned.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
---
 include/exec/memop.h | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/include/exec/memop.h b/include/exec/memop.h
index 25d027434a..04e4048f0b 100644
--- a/include/exec/memop.h
+++ b/include/exec/memop.h
@@ -81,6 +81,42 @@ typedef enum MemOp {
     MO_ALIGN_32 = 5 << MO_ASHIFT,
     MO_ALIGN_64 = 6 << MO_ASHIFT,
 
+    /*
+     * MO_ATOM_* describes that atomicity requirements of the operation:
+     * MO_ATOM_IFALIGN: the operation must be single-copy atomic if and
+     *    only if it is aligned; if unaligned there is no atomicity.
+     * MO_ATOM_NONE: the operation has no atomicity requirements.
+     * MO_ATOM_SUBALIGN: the operation is single-copy atomic by parts
+     *    by the alignment.  E.g. if the address is 0 mod 4, then each
+     *    4-byte subobject is single-copy atomic.
+     *    This is the atomicity of IBM Power and S390X processors.
+     * MO_ATOM_WITHIN16: the operation is single-copy atomic, even if it
+     *    is unaligned, so long as it does not cross a 16-byte boundary;
+     *    if it crosses a 16-byte boundary there is no atomicity.
+     *    This is the atomicity of Arm FEAT_LSE2.
+     *
+     * MO_ATMAX_* describes the maximum atomicity unit required:
+     * MO_ATMAX_SIZE: the entire operation, i.e. MO_SIZE.
+     * MO_ATMAX_[248]: units of N bytes.
+     *
+     * Note the default (i.e. 0) values are single-copy atomic to the
+     * size of the operation, if aligned.  This retains the behaviour
+     * from before these were introduced.
+     */
+    MO_ATOM_SHIFT    = 8,
+    MO_ATOM_MASK     = 0x3 << MO_ATOM_SHIFT,
+    MO_ATOM_IFALIGN  = 0 << MO_ATOM_SHIFT,
+    MO_ATOM_NONE     = 1 << MO_ATOM_SHIFT,
+    MO_ATOM_SUBALIGN = 2 << MO_ATOM_SHIFT,
+    MO_ATOM_WITHIN16 = 3 << MO_ATOM_SHIFT,
+
+    MO_ATMAX_SHIFT = 10,
+    MO_ATMAX_MASK  = 0x3 << MO_ATMAX_SHIFT,
+    MO_ATMAX_SIZE  = 0 << MO_ATMAX_SHIFT,
+    MO_ATMAX_2     = 1 << MO_ATMAX_SHIFT,
+    MO_ATMAX_4     = 2 << MO_ATMAX_SHIFT,
+    MO_ATMAX_8     = 3 << MO_ATMAX_SHIFT,
+
     /* Combinations of the above, for ease of use.  */
     MO_UB    = MO_8,
     MO_UW    = MO_16,
-- 
2.34.1


