Re: [PATCH] hw/arm/armv7m: Disable reentrancy guard for v7m_sysreg_ns_ops MRs

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

On 14/11/25 16:53, Peter Maydell wrote:
> 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>
> ---
>   hw/arm/armv7m.c | 12 ++++++++++++
>   1 file changed, 12 insertions(+)

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>