Re: [Qemu-devel] [PATCH v2 4/8] linux-user: arm: handle CPSR.E correctly in strex emulation

[Paolo Bonzini]

Il 03/06/2014 11:54, Peter Maydell ha scritto:
> In user emulation, things are more complicated for BE32,
> because we're sort of emulating the word-invariant
> bigendian using byte-invariant big-endian (this is
> safe because there's no way for a userspace program
> to get at anything that would let it tell the
> difference). So we can't just say "set SCTLR.B
> and handle as if SCTLR.B is set in the way system
> emulation would", because the behaviour has to
> be different.
> 
> So in summary I'm not sure of the right approach
> any more...

I think overall sctlr_b makes for more accurate and overall
clearer code.

Here are the functions I'm using to map between the various properties:

+static inline bool bswap_code(bool sctlr_b)
+{
+#ifdef CONFIG_USER_ONLY
+    /* Mixed-endian modes are BE8 (SCTLR.B = 0, TARGET_WORDS_BIGENDIAN = 1)
+     * and LE8 (SCTLR.B = 1, TARGET_WORDS_BIGENDIAN = 0).
+     */
+    return
+#ifdef TARGET_WORDS_BIGENDIAN
+        1 ^
+#endif
+        sctlr_b;
+#else
+    /* We do not implement BE32 mode for system-mode emulation, but
+     * anyway it would always do little-endian accesses with
+     * TARGET_WORDS_BIGENDIAN = 0.
+     */
+    return 0;
+#endif
+}
+
+#ifdef CONFIG_USER_ONLY
+static inline bool arm_cpu_bswap_data(CPUARMState *env)
+{
+    return
+#ifdef TARGET_WORDS_BIGENDIAN
+       1 ^
+#endif
+       !!(env->cp15.c1_sys & SCTLR_B) ^
+       !!(env->uncached_cpsr & CPSR_E);
+}
+#endif
+
+static inline bool arm_tbflag_is_data_be(unsigned tbflags)
+{
+    return
+#ifdef CONFIG_USER_ONLY
+        ARM_TBFLAG_SCTLR_B(tbflags) ^
+#endif
+        ARM_TBFLAG_CPSR_E(tbflags);
+}
+

I think this is reasonably close to what you would have for SCTLR.B
emulation, only the XORing of addresses is missing.

bswap_code is used in much fewer places than the current env->bswap_code,
basically only in the definitions of arm_ld*_code and get_user_code_*.
Everywhere else the code is accessing SCTLR.B, which is "real"
architectural state.  The confusing manner of handling it in user-mode
emulation is wrapped by the above three inline functions.

Paolo