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Re: [Qemu-arm] [PATCH 4/4] arm: Fix APSR writes via M profile MSR
From: |
Alex Bennée |
Subject: |
Re: [Qemu-arm] [PATCH 4/4] arm: Fix APSR writes via M profile MSR |
Date: |
Mon, 20 Mar 2017 11:01:54 +0000 |
User-agent: |
mu4e 0.9.19; emacs 25.2.10 |
Peter Maydell <address@hidden> 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 <address@hidden>
Reviewed-by: Alex Bennée <address@hidden>
> ---
> 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
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