Re: [Qemu-devel] [PATCH 04/18] armv7m: Explicit error for bad vector table

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

On 9 November 2015 at 01:11, Michael Davidsaver <mdavidsaver@gmail.com> wrote:
> Give an explicit error and abort when a load
> from VECBASE fails.  Otherwise would likely
> jump to 0, which for v7-m holds the reset stack
> pointer address.
>
> Signed-off-by: Michael Davidsaver <mdavidsaver@gmail.com>
> ---
>  target-arm/helper.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/target-arm/helper.c b/target-arm/helper.c
> index 4178400..1d7ac43 100644
> --- a/target-arm/helper.c
> +++ b/target-arm/helper.c
> @@ -5496,7 +5496,17 @@ void arm_v7m_cpu_do_interrupt(CPUState *cs)
>      /* Clear IT bits */
>      env->condexec_bits = 0;
>      env->regs[14] = lr;
> -    addr = ldl_phys(cs->as, env->v7m.vecbase + env->v7m.exception * 4);
> +    {
> +        MemTxResult result;
> +        addr = address_space_ldl(cs->as,
> +                                 env->v7m.vecbase + env->v7m.exception * 4,
> +                                 MEMTXATTRS_UNSPECIFIED, &result);
> +        if (result != MEMTX_OK) {
> +            cpu_abort(cs, "Failed to read from exception vector table "
> +                      "entry %08x\n",
> +                      env->v7m.vecbase + env->v7m.exception * 4);
> +        }
> +    }

The behaviour on a failed vector table read is actually architecturally
specified: we should take a nested exception (escalated to HardFault).
If it happens while we're trying to take a HardFault in the first place
then we go into Lockup (where the CPU sits around repeatedly trying
to execute an instruction at 0xFFFFFFFE; it is technically possible
to get back out of Lockup by taking an NMI or a system reset).

That said, trying to get nested exceptions and priority escalation
right is fairly involved, and implementing lockup is both involved
and an exercise in pointlessness. So I think this code is an
improvement overall. I would suggest some small changes, though:

(1) factor this out into its own function, something like:
static uint32_t v7m_read_vector(CPUARMState *env, int excnum)
so the calling code can just do
   addr = v7m_read_vector(env, env->v7m.exception);
(2) use a local variable for "env->v7m.vecbase + excnum * 4"
rather than calculating it twice

thanks
-- PMM