arm: Rewrite ERET to behave according to the ARMv8 ARM

The ERET instruction doesn't set PSTATE correctly in some cases
(particularly when returning to aarch32 code). Among other things,
this breaks EL0 thumb code when using a 64-bit kernel. This changeset
updates the ERET implementation to match the ARM ARM.

Change-Id: I408e7c69a23cce437859313dfe84e68744b07c98
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Nathanael Premillieu <nathanael.premillieu@arm.com>

2 files changed, 120 insertions, 0 deletions

diff --git a/src/arch/arm/insts/static_inst.cc b/src/arch/arm/insts/static_inst.cc
index d4ea1bcdf..3defc07c2 100644
--- a/src/arch/arm/insts/static_inst.cc
+++ b/src/arch/arm/insts/static_inst.cc
@@ -727,4 +727,115 @@ ArmStaticInst::checkAdvSIMDOrFPEnabled32(ThreadContext *tc,
 }
 
 
+static uint8_t
+getRestoredITBits(ThreadContext *tc, CPSR spsr)
+{
+    // See: shared/functions/system/RestoredITBits in the ARM ARM
+
+    const ExceptionLevel el = opModeToEL((OperatingMode) (uint8_t)spsr.mode);
+    const uint8_t it = itState(spsr);
+
+    if (!spsr.t || spsr.il)
+        return 0;
+
+    // The IT bits are forced to zero when they are set to a reserved
+    // value.
+    if (bits(it, 7, 4) != 0 && bits(it, 3, 0) == 0)
+        return 0;
+
+    const bool itd = el == EL2 ?
+        ((SCTLR)tc->readMiscReg(MISCREG_HSCTLR)).itd :
+        ((SCTLR)tc->readMiscReg(MISCREG_SCTLR)).itd;
+
+    // The IT bits are forced to zero when returning to A32 state, or
+    // when returning to an EL with the ITD bit set to 1, and the IT
+    // bits are describing a multi-instruction block.
+    if (itd && bits(it, 2, 0) != 0)
+        return 0;
+
+    return it;
+}
+
+static bool
+illegalExceptionReturn(ThreadContext *tc, CPSR cpsr, CPSR spsr)
+{
+    const OperatingMode mode = (OperatingMode) (uint8_t)spsr.mode;
+    if (badMode(mode))
+        return true;
+
+    const OperatingMode cur_mode = (OperatingMode) (uint8_t)cpsr.mode;
+    const ExceptionLevel target_el = opModeToEL(mode);
+    if (target_el > opModeToEL(cur_mode))
+        return true;
+
+    if (target_el == EL3 && !ArmSystem::haveSecurity(tc))
+        return true;
+
+    if (target_el == EL2 && !ArmSystem::haveVirtualization(tc))
+        return true;
+
+    if (!spsr.width) {
+        // aarch64
+        if (!ArmSystem::highestELIs64(tc))
+            return true;
+
+        if (spsr & 0x2)
+            return true;
+        if (target_el == EL0 && spsr.sp)
+            return true;
+        if (target_el == EL2 && !((SCR)tc->readMiscReg(MISCREG_SCR_EL3)).ns)
+            return false;
+    } else {
+        return badMode32(mode);
+    }
+
+    return false;
+}
+
+CPSR
+ArmStaticInst::getPSTATEFromPSR(ThreadContext *tc, CPSR cpsr, CPSR spsr) const
+{
+    CPSR new_cpsr = 0;
+
+    // gem5 doesn't implement single-stepping, so force the SS bit to
+    // 0.
+    new_cpsr.ss = 0;
+
+    if (illegalExceptionReturn(tc, cpsr, spsr)) {
+        new_cpsr.il = 1;
+    } else {
+        new_cpsr.il = spsr.il;
+        if (spsr.width && badMode32((OperatingMode)(uint8_t)spsr.mode)) {
+            new_cpsr.il = 1;
+        } else if (spsr.width) {
+            new_cpsr.mode = spsr.mode;
+        } else {
+            new_cpsr.el = spsr.el;
+            new_cpsr.sp = spsr.sp;
+        }
+    }
+
+    new_cpsr.nz = spsr.nz;
+    new_cpsr.c = spsr.c;
+    new_cpsr.v = spsr.v;
+    if (new_cpsr.width) {
+        // aarch32
+        const ITSTATE it = getRestoredITBits(tc, spsr);
+        new_cpsr.q = spsr.q;
+        new_cpsr.ge = spsr.ge;
+        new_cpsr.e = spsr.e;
+        new_cpsr.aif = spsr.aif;
+        new_cpsr.t = spsr.t;
+        new_cpsr.it2 = it.top6;
+        new_cpsr.it1 = it.bottom2;
+    } else {
+        // aarch64
+        new_cpsr.daif = spsr.daif;
+    }
+
+    return new_cpsr;
+}
+
+
+
 }
diff --git a/src/arch/arm/insts/static_inst.hh b/src/arch/arm/insts/static_inst.hh
index 9ca64d1fe..55d16f69d 100644
--- a/src/arch/arm/insts/static_inst.hh
+++ b/src/arch/arm/insts/static_inst.hh
@@ -404,6 +404,15 @@ class ArmStaticInst : public StaticInst
                                     NSACR nsacr, FPEXC fpexc,
                                     bool fpexc_check, bool advsimd) const;
 
+    /**
+     * Get the new PSTATE from a SPSR register in preparation for an
+     * exception return.
+     *
+     * See shared/functions/system/SetPSTATEFromPSR in the ARM ARM
+     * psueodcode library.
+     */
+    CPSR getPSTATEFromPSR(ThreadContext *tc, CPSR cpsr, CPSR spsr) const;
+
   public:
     virtual void
     annotateFault(ArmFault *fault) {}