target/arm: Fix routing of singlestep exceptions

When generating an architectural single-step exception we were
routing it to the "default exception level", which is to say
the same exception level we execute at except that EL0 exceptions
go to EL1. This is incorrect because the debug exception level
can be configured by the guest for situations such as single
stepping of EL0 and EL1 code by EL2.

We have to track the target debug exception level in the TB
flags, because it is dependent on CPU state like HCR_EL2.TGE
and MDCR_EL2.TDE. (That we were previously calling the
arm_debug_target_el() function to determine dc->ss_same_el
is itself a bug, though one that would only have manifested
as incorrect syndrome information.) Since we are out of TB
flag bits unless we want to expand into the cs_base field,
we share some bits with the M-profile only HANDLER and
STACKCHECK bits, since only A-profile has this singlestep.

Fixes: https://bugs.launchpad.net/qemu/+bug/1838913
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20190805130952.4415-3-peter.maydell@linaro.org

5 files changed, 26 insertions, 6 deletions

diff --git a/target/arm/cpu.h b/target/arm/cpu.h
index 67f2af0..d12c746 100644
--- a/target/arm/cpu.h
+++ b/target/arm/cpu.h
@@ -3148,6 +3148,11 @@ FIELD(TBFLAG_ANY, PSTATE_SS, 26, 1)
 /* Target EL if we take a floating-point-disabled exception */
 FIELD(TBFLAG_ANY, FPEXC_EL, 24, 2)
 FIELD(TBFLAG_ANY, BE_DATA, 23, 1)
+/*
+ * For A-profile only, target EL for debug exceptions.
+ * Note that this overlaps with the M-profile-only HANDLER and STACKCHECK bits.
+ */
+FIELD(TBFLAG_ANY, DEBUG_TARGET_EL, 21, 2)
 
 /* Bit usage when in AArch32 state: */
 FIELD(TBFLAG_A32, THUMB, 0, 1)
diff --git a/target/arm/helper.c b/target/arm/helper.c
index b74c23a..24806c1 100644
--- a/target/arm/helper.c
+++ b/target/arm/helper.c
@@ -11170,6 +11170,12 @@ void cpu_get_tb_cpu_state(CPUARMState *env, target_ulong *pc,
         }
     }
 
+    if (!arm_feature(env, ARM_FEATURE_M)) {
+        int target_el = arm_debug_target_el(env);
+
+        flags = FIELD_DP32(flags, TBFLAG_ANY, DEBUG_TARGET_EL, target_el);
+    }
+
     *pflags = flags;
     *cs_base = 0;
 }
diff --git a/target/arm/translate-a64.c b/target/arm/translate-a64.c
index f6729b9..90850ea 100644
--- a/target/arm/translate-a64.c
+++ b/target/arm/translate-a64.c
@@ -14180,7 +14180,7 @@ static void aarch64_tr_init_disas_context(DisasContextBase *dcbase,
     dc->ss_active = FIELD_EX32(tb_flags, TBFLAG_ANY, SS_ACTIVE);
     dc->pstate_ss = FIELD_EX32(tb_flags, TBFLAG_ANY, PSTATE_SS);
     dc->is_ldex = false;
-    dc->ss_same_el = (arm_debug_target_el(env) == dc->current_el);
+    dc->debug_target_el = FIELD_EX32(tb_flags, TBFLAG_ANY, DEBUG_TARGET_EL);
 
     /* Bound the number of insns to execute to those left on the page.  */
     bound = -(dc->base.pc_first | TARGET_PAGE_MASK) / 4;
diff --git a/target/arm/translate.c b/target/arm/translate.c
index 19b9d8f..b32508c 100644
--- a/target/arm/translate.c
+++ b/target/arm/translate.c
@@ -11882,7 +11882,9 @@ static void arm_tr_init_disas_context(DisasContextBase *dcbase, CPUState *cs)
     dc->ss_active = FIELD_EX32(tb_flags, TBFLAG_ANY, SS_ACTIVE);
     dc->pstate_ss = FIELD_EX32(tb_flags, TBFLAG_ANY, PSTATE_SS);
     dc->is_ldex = false;
-    dc->ss_same_el = false; /* Can't be true since EL_d must be AArch64 */
+    if (!arm_feature(env, ARM_FEATURE_M)) {
+        dc->debug_target_el = FIELD_EX32(tb_flags, TBFLAG_ANY, DEBUG_TARGET_EL);
+    }
 
     dc->page_start = dc->base.pc_first & TARGET_PAGE_MASK;
 
diff --git a/target/arm/translate.h b/target/arm/translate.h
index 4505319..b65954c 100644
--- a/target/arm/translate.h
+++ b/target/arm/translate.h
@@ -50,6 +50,8 @@ typedef struct DisasContext {
     uint32_t svc_imm;
     int aarch64;
     int current_el;
+    /* Debug target exception level for single-step exceptions */
+    int debug_target_el;
     GHashTable *cp_regs;
     uint64_t features; /* CPU features bits */
     /* Because unallocated encodings generate different exception syndrome
@@ -70,8 +72,6 @@ typedef struct DisasContext {
      * ie A64 LDX*, LDAX*, A32/T32 LDREX*, LDAEX*.
      */
     bool is_ldex;
-    /* True if a single-step exception will be taken to the current EL */
-    bool ss_same_el;
     /* True if v8.3-PAuth is active.  */
     bool pauth_active;
     /* True with v8.5-BTI and SCTLR_ELx.BT* set.  */
@@ -251,8 +251,15 @@ static inline void gen_exception(int excp, uint32_t syndrome,
 /* Generate an architectural singlestep exception */
 static inline void gen_swstep_exception(DisasContext *s, int isv, int ex)
 {
-    gen_exception(EXCP_UDEF, syn_swstep(s->ss_same_el, isv, ex),
-                  default_exception_el(s));
+    bool same_el = (s->debug_target_el == s->current_el);
+
+    /*
+     * If singlestep is targeting a lower EL than the current one,
+     * then s->ss_active must be false and we can never get here.
+     */
+    assert(s->debug_target_el >= s->current_el);
+
+    gen_exception(EXCP_UDEF, syn_swstep(same_el, isv, ex), s->debug_target_el);
 }
 
 /*