[PULL 3/4] hw/arm/armv7m: Disable reentrancy guard for v7m_sysreg_ns_ops MRs

[Peter Maydell <peter.maydell@linaro.org>]

For M-profile cores which support TrustZone, there are some memory
areas which are "NS aliases" -- a Secure access to these addresses
really performs an NS access to a different part of the device.  We
implement these using MemoryRegionOps read and write functions which
pass the access on with adjusted attributes using
memory_region_dispatch_read() and memory_region_dispatch_write().

Since the MR we are dispatching to is owned by the same device that
owns the NS-alias MR (the TYPE_ARMV7M container object), this trips
the reentrancy-guard that is applied by access_with_adjusted_size().

Mark the NS alias MemoryRegions as disable_reentrancy_guard; this is
safe because v7m_sysreg_ns_read() and v7m_sysreg_ns_write() do not
touch any of the device's state.  (Any further reentrancy attempts by
the underlying MR will still be caught.)

Without this fix, an attempt to read from an address like 0xe002e010,
which is a register in the NS systick alias, will fail and provoke

 qemu-system-arm: warning: Blocked re-entrant IO on MemoryRegion: v7m_systick at addr: 0x0

We didn't notice this earlier because almost all code accesses
the registers and systick via the non-alias addresses; the NS
aliases are only need for the rarer case of Secure code that needs
to manage the NS timer or system state on behalf of NS code.

Note that although the v7m_systick_ops read and write functions
also call memory_region_dispatch_{read,write}, this MR does not
need to have the reentrancy-guard disabled because the underlying
MR that it forwards to is owned by a different device (the
TYPE_SYSTICK timer device).

Reported via a stackoverflow question:
https://stackoverflow.com/questions/79808107/what-this-error-is-even-about-qemu-system-arm-warning-blocked-re-entrant-io

Cc: qemu-stable@nongnu.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20251114155304.2662414-1-peter.maydell@linaro.org
---
 hw/arm/armv7m.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/hw/arm/armv7m.c b/hw/arm/armv7m.c
index c4a9c3ac529..7fa1b37630e 100644
--- a/hw/arm/armv7m.c
+++ b/hw/arm/armv7m.c
@@ -442,6 +442,12 @@ static void armv7m_realize(DeviceState *dev, Error **errp)
                               &v7m_sysreg_ns_ops,
                               sysbus_mmio_get_region(sbd, 0),
                               "nvic_sysregs_ns", 0x1000);
+        /*
+         * This MR calls memory_region_dispatch_read/write to access the
+         * real region for the NVIC sysregs (which is also owned by this
+         * device), so reentrancy through here is expected and safe.
+         */
+        s->sysreg_ns_mem.disable_reentrancy_guard = true;
         memory_region_add_subregion(&s->container, 0xe002e000,
                                     &s->sysreg_ns_mem);
     }
@@ -499,6 +505,12 @@ static void armv7m_realize(DeviceState *dev, Error **errp)
         memory_region_init_io(&s->systick_ns_mem, OBJECT(s),
                               &v7m_sysreg_ns_ops, &s->systickmem,
                               "v7m_systick_ns", 0xe0);
+        /*
+         * This MR calls memory_region_dispatch_read/write to access the
+         * real region for the systick regs (which is also owned by this
+         * device), so reentrancy through here is expected and safe.
+         */
+        s->systick_ns_mem.disable_reentrancy_guard = true;
         memory_region_add_subregion_overlap(&s->container, 0xe002e010,
                                             &s->systick_ns_mem, 1);
     }
-- 
2.43.0