[PATCH] target/arm: honour CCR.BFHFNMIGN for probed data BusFaults

[Kyle Fox <kylefoxaustin.github@gmail.com>]

M-profile CCR.BFHFNMIGN lets software executing at a negative execution
priority (in HardFault/NMI, or with FAULTMASK set) suppress precise data
BusFaults caused by load/store instructions: the access completes
returning UNKNOWN data, the fault status is recorded in BFSR/BFAR, but no
BusFault exception is taken. Software uses this to probe for the presence
of a device.

QEMU stored CCR.BFHFNMIGN but never consumed it: arm_cpu_do_transaction_
failed() always raised the external abort, which arm_v7m_cpu_do_interrupt()
pended as a BusFault and then escalated to a HardFault it could not take at
priority -1, aborting the VM with "Lockup: can't escalate 3 to HardFault".

Honour the bit in arm_cpu_do_transaction_failed(): when the access is a
data access from M-profile code at negative priority with BFHFNMIGN set,
record PRECISERR/BFARVALID and BFAR and return without raising, so the
faulting instruction completes instead of re-faulting forever. Instruction
fetches are unaffected, since BFHFNMIGN applies only to data accesses.

This surfaced running the real NXP i.MX 95 System Manager firmware on the
emulated Cortex-M33: its SystemMemoryProbe() (set BFHFNMIGN + FAULTMASK,
do the access, test CFSR.BFARVALID) locked up the VM. With this change the
SM's debug-monitor memory-probe commands run and recover correctly.

Signed-off-by: Kyle Fox <kylefoxaustin.github@gmail.com>
---
Found while bringing up an out-of-tree i.MX 95 machine running the real NXP
System Manager firmware on the emulated Cortex-M33; the change is generic to
any ARMv7-M guest that probes for devices via BusFault suppression. It is
independent of (and posted alongside) a separate PMSAv7 MPU align-down fix
from the same bring-up.

The new path only runs for an M-profile data access at negative priority with
CCR.BFHFNMIGN set - the previously-broken case that aborted the VM. Normal
BusFaults (no BFHFNMIGN, or at non-negative priority) and instruction fetches
are unchanged.

Tested on master: qemu-system-arm builds clean, and the ARMv7-M / MPS2 qtests
pass with no regression -- boot-serial (incl. stm32vldiscovery, Cortex-M3),
the stm32l4x5 suite (Cortex-M4: exti/gpio/rcc/syscfg/usart), microbit,
sse-timer and cmsdk-apb-watchdog.

 target/arm/tcg/tlb_helper.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/target/arm/tcg/tlb_helper.c b/target/arm/tcg/tlb_helper.c
index f90765cb59..cbef9cb03e 100644
--- a/target/arm/tcg/tlb_helper.c
+++ b/target/arm/tcg/tlb_helper.c
@@ -10,6 +10,7 @@
 #include "helper.h"
 #include "internals.h"
 #include "cpu-features.h"
+#include "hw/intc/armv7m_nvic.h"
 
 /*
  * Returns true if the stage 1 translation regime is using LPAE format page
@@ -318,8 +319,31 @@ void arm_cpu_do_transaction_failed(CPUState *cs, hwaddr physaddr,
                                    MemTxResult response, uintptr_t retaddr)
 {
     ARMCPU *cpu = ARM_CPU(cs);
+    CPUARMState *env = &cpu->env;
     ARMMMUFaultInfo fi = {};
 
+    /*
+     * For M-profile, CCR.BFHFNMIGN lets software executing at a negative
+     * priority (in HardFault/NMI, or with FAULTMASK set) suppress precise
+     * data BusFaults from load/store instructions: the access completes
+     * returning UNKNOWN data (the store is dropped), the fault status is
+     * recorded in BFSR/BFAR, but no BusFault exception is taken. This is
+     * the mechanism software uses to probe for the presence of a device
+     * (e.g. the NXP System Manager's SystemMemoryProbe). Honour it by
+     * recording the status and returning without raising, so the faulting
+     * instruction completes rather than re-faulting forever. BFHFNMIGN
+     * applies only to data accesses, so instruction fetches are unaffected.
+     */
+    if (arm_feature(env, ARM_FEATURE_M) &&
+        access_type != MMU_INST_FETCH &&
+        (env->v7m.ccr[M_REG_NS] & R_V7M_CCR_BFHFNMIGN_MASK) &&
+        armv7m_nvic_neg_prio_requested(env->nvic, env->v7m.secure)) {
+        env->v7m.cfsr[M_REG_NS] |=
+            (R_V7M_CFSR_PRECISERR_MASK | R_V7M_CFSR_BFARVALID_MASK);
+        env->v7m.bfar = addr;
+        return;
+    }
+
     /* now we have a real cpu fault */
     cpu_restore_state(cs, retaddr);
 
-- 
2.34.1