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References: <1506092407-26985-1-git-send-email-peter.maydell@linaro.org>
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From: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <0136d727-d125-7eed-ca13-02011cbdf921@linaro.org>
Date: Thu, 5 Oct 2017 12:32:27 -0400
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Subject: Re: [Qemu-devel] [PATCH 08/20] target/arm: Don't warn about
 exception return with PC low bit set for v8M
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To: Peter Maydell <peter.maydell@linaro.org>, qemu-arm@nongnu.org, qemu-devel@nongnu.org
Cc: patches@linaro.org

On 09/22/2017 10:59 AM, Peter Maydell wrote:
> +        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;

If you're going to always test regs[15] & 1, you might as well use that and
avoid an unlikely(?) writeback.

    if (env->regs[15] & 1) {
        if (!arm_feature(env, ARM_FEATURE_V8)) {
            qemu_log_mask(...);
        }
        env->regs[15] &= ~1U;
    }

Otherwise,
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>


r~