Re: [PATCH v3 23/29] bsd-user/arm/target_arch_signal.h: arm set_mcontext

[Warner Losh]

On Thu, Nov 4, 2021 at 12:43 PM Richard Henderson <richard.henderson@linaro.org> wrote:

On 11/4/21 10:05 AM, Warner Losh wrote:
> +    /*
> +     * Make sure T mode matches the PC's notion of thumb mode, although
> +     * FreeBSD lets the processor sort this out, so we may need remove
> +     * this check, or generate a signal...
> +     */
> +    if (!!(tswap32(gr[TARGET_REG_PC]) & 1) != !!(cpsr & CPSR_T)) {
> +        return -TARGET_EINVAL;
> +    }

I've had a read through the Arm ARM for "movs pc, lr", which is how swi_exit returns to
userspace:

     CPSRWriteByInstr(SPSR[], '1111', TRUE);
     ...
     BranchWritePC(result);

So the CPSR gets written first, which sets the T bit, and thus the result of
CurrentInstrSet(), then

BranchWritePC(bits(32) address)
   if CurrentInstrSet() == InstrSet_ARM then
     if ArchVersion() < 6 && address<1:0> != '00' then UNPREDICTABLE;
     BranchTo(address<31:2>:'00');
   ...
   else
     BranchTo(address<31:1>:'0');

> +    env->regs[15] = tswap32(gr[TARGET_REG_PC]);

So this should mask the low 1 or 2 bits depending on cpsr & CPSR_T.

Will do. Thanks for all the patient explanations.

Warner

 

r~