Re: [Qemu-devel] [PATCH 08/20] target/arm: Don't warn about exception return with PC low bit set for v8M

["Philippe Mathieu-Daudé" <f4bug@amsat.org>]

On 09/22/2017 11:59 AM, Peter Maydell wrote:
> In the v8M architecture, return from an exception to a PC which
> has bit 0 set is not UNPREDICTABLE; it is defined that bit 0
> is discarded [R_HRJH]. Restrict our complaint about this to v7M.
> 
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

> ---
>   target/arm/helper.c | 20 +++++++++++++-------
>   1 file changed, 13 insertions(+), 7 deletions(-)
> 
> diff --git a/target/arm/helper.c b/target/arm/helper.c
> index 979129e..59a07d2 100644
> --- a/target/arm/helper.c
> +++ b/target/arm/helper.c
> @@ -6421,16 +6421,22 @@ static void do_v7m_exception_exit(ARMCPU *cpu)
>           env->regs[12] = ldl_phys(cs->as, frameptr + 0x10);
>           env->regs[14] = ldl_phys(cs->as, frameptr + 0x14);
>           env->regs[15] = ldl_phys(cs->as, frameptr + 0x18);
> -        if (env->regs[15] & 1) {
> +
> +        /* Returning from an exception with a PC with bit 0 set is defined
> +         * behaviour on v8M (bit 0 is ignored), but for v7M it was specified
> +         * to be UNPREDICTABLE. In practice actual v7M hardware seems to ignore
> +         * the lsbit, and there are several RTOSes out there which incorrectly
> +         * assume the r15 in the stack frame should be a Thumb-style "lsbit
> +         * indicates ARM/Thumb" value, so ignore the bit on v7M as well, but
> +         * complain about the badly behaved guest.
> +         */
> +        if ((env->regs[15] & 1) && !arm_feature(env, ARM_FEATURE_V8)) {
>               qemu_log_mask(LOG_GUEST_ERROR,
>                             "M profile return from interrupt with misaligned "
> -                          "PC is UNPREDICTABLE\n");
> -            /* Actual hardware seems to ignore the lsbit, and there are several
> -             * RTOSes out there which incorrectly assume the r15 in the stack
> -             * frame should be a Thumb-style "lsbit indicates ARM/Thumb" value.
> -             */
> -            env->regs[15] &= ~1U;
> +                          "PC is UNPREDICTABLE on v7M\n");
>           }
> +        env->regs[15] &= ~1U;
> +
>           xpsr = ldl_phys(cs->as, frameptr + 0x1c);
>   
>           if (arm_feature(env, ARM_FEATURE_V8)) {
>