Re: [PATCH 4/4] arm: Fix APSR writes via M profile MSR

["Alex Bennée" <alex.bennee@linaro.org>]

Peter Maydell <peter.maydell@linaro.org> writes:

> Our implementation of writes to the APSR for M-profile via the MSR
> instruction was badly broken.
>
> First and worst, we had the sense wrong on the test of bit 2 of the
> SYSm field -- this is supposed to request an APSR write if bit 2 is 0
> but we were doing it if bit 2 was 1.  This bug was introduced in
> commit 58117c9bb429cd, so hasn't been in a QEMU release.
>
> Secondly, the choice of exactly which parts of APSR should be written
> is defined by bits in the 'mask' field.  We were not passing these
> through from instruction decode, making it impossible to check them
> in the helper.
>
> Pass the mask bits through from the instruction decode to the helper
> function and process them appropriately; fix the wrong sense of the
> SYSm bit 2 check.
>
> Invalid mask values and invalid combinations of mask and register
> number are UNPREDICTABLE; we choose to treat them as if the mask
> values were valid.
>
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

> ---
>  target/arm/helper.c    | 26 ++++++++++++++++++++++----
>  target/arm/translate.c |  3 ++-
>  2 files changed, 24 insertions(+), 5 deletions(-)
>
> diff --git a/target/arm/helper.c b/target/arm/helper.c
> index 948aba2..8349e1f 100644
> --- a/target/arm/helper.c
> +++ b/target/arm/helper.c
> @@ -8478,8 +8478,18 @@ uint32_t HELPER(v7m_mrs)(CPUARMState *env, uint32_t reg)
>      }
>  }
>
> -void HELPER(v7m_msr)(CPUARMState *env, uint32_t reg, uint32_t val)
> -{
> +void HELPER(v7m_msr)(CPUARMState *env, uint32_t maskreg, uint32_t val)
> +{
> +    /* We're passed bits [11..0] of the instruction; extract
> +     * SYSm and the mask bits.
> +     * Invalid combinations of SYSm and mask are UNPREDICTABLE;
> +     * we choose to treat them as if the mask bits were valid.
> +     * NB that the pseudocode 'mask' variable is bits [11..10],
> +     * whereas ours is [11..8].
> +     */
> +    uint32_t mask = extract32(maskreg, 8, 4);
> +    uint32_t reg = extract32(maskreg, 0, 8);
> +
>      if (arm_current_el(env) == 0 && reg > 7) {
>          /* only xPSR sub-fields may be written by unprivileged */
>          return;
> @@ -8488,8 +8498,16 @@ void HELPER(v7m_msr)(CPUARMState *env, uint32_t reg, uint32_t val)
>      switch (reg) {
>      case 0 ... 7: /* xPSR sub-fields */
>          /* only APSR is actually writable */
> -        if (reg & 4) {
> -            xpsr_write(env, val, 0xf8000000); /* APSR */
> +        if (!(reg & 4)) {
> +            uint32_t apsrmask = 0;
> +
> +            if (mask & 8) {
> +                apsrmask |= 0xf8000000; /* APSR NZCVQ */
> +            }
> +            if ((mask & 4) && arm_feature(env, ARM_FEATURE_THUMB_DSP)) {
> +                apsrmask |= 0x000f0000; /* APSR GE[3:0] */
> +            }
> +            xpsr_write(env, val, apsrmask);
>          }
>          break;
>      case 8: /* MSP */
> diff --git a/target/arm/translate.c b/target/arm/translate.c
> index 9090356..ce7f19f 100644
> --- a/target/arm/translate.c
> +++ b/target/arm/translate.c
> @@ -10391,7 +10391,8 @@ static int disas_thumb2_insn(CPUARMState *env, DisasContext *s, uint16_t insn_hw
>                      case 0: /* msr cpsr.  */
>                          if (arm_dc_feature(s, ARM_FEATURE_M)) {
>                              tmp = load_reg(s, rn);
> -                            addr = tcg_const_i32(insn & 0xff);
> +                            /* the constant is the mask and SYSm fields */
> +                            addr = tcg_const_i32(insn & 0xfff);
>                              gen_helper_v7m_msr(cpu_env, addr, tmp);
>                              tcg_temp_free_i32(addr);
>                              tcg_temp_free_i32(tmp);


--
Alex Bennée