[qemu-xen staging-4.13] target/arm: Allow reading flags from FPSCR for M-profile

[patchbot@xxxxxxx]

commit cdc6896659b85f7ed8f7552850312e55170de0c5
Author:     Christophe Lyon <christophe.lyon@xxxxxxxxxx>
AuthorDate: Fri Oct 25 11:57:11 2019 +0200
Commit:     Michael Roth <mdroth@xxxxxxxxxxxxxxxxxx>
CommitDate: Mon Nov 4 08:34:45 2019 -0600

    target/arm: Allow reading flags from FPSCR for M-profile
    
    rt==15 is a special case when reading the flags: it means the
    destination is APSR. This patch avoids rejecting
    vmrs apsr_nzcv, fpscr
    as illegal instruction.
    
    Cc: qemu-stable@xxxxxxxxxx
    Signed-off-by: Christophe Lyon <christophe.lyon@xxxxxxxxxx>
    Message-id: 20191025095711.10853-1-christophe.lyon@xxxxxxxxxx
    [PMM: updated the comment]
    Reviewed-by: Peter Maydell <peter.maydell@xxxxxxxxxx>
    Signed-off-by: Peter Maydell <peter.maydell@xxxxxxxxxx>
    (cherry picked from commit 2529ab43b8a05534494704e803e0332d111d8b91)
    Signed-off-by: Michael Roth <mdroth@xxxxxxxxxxxxxxxxxx>
---
 target/arm/translate-vfp.inc.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/target/arm/translate-vfp.inc.c b/target/arm/translate-vfp.inc.c
index ef45cecbea..75406fd9db 100644
--- a/target/arm/translate-vfp.inc.c
+++ b/target/arm/translate-vfp.inc.c
@@ -704,9 +704,10 @@ static bool trans_VMSR_VMRS(DisasContext *s, arg_VMSR_VMRS 
*a)
     if (arm_dc_feature(s, ARM_FEATURE_M)) {
         /*
          * The only M-profile VFP vmrs/vmsr sysreg is FPSCR.
-         * Writes to R15 are UNPREDICTABLE; we choose to undef.
+         * Accesses to R15 are UNPREDICTABLE; we choose to undef.
+         * (FPSCR -> r15 is a special case which writes to the PSR flags.)
          */
-        if (a->rt == 15 || a->reg != ARM_VFP_FPSCR) {
+        if (a->rt == 15 && (!a->l || a->reg != ARM_VFP_FPSCR)) {
             return false;
         }
     }
--
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