Re: [Qemu-devel] [PATCH v2 06/13] hw/misc/tz-mpc.c: Implement registers

[Peter Maydell]

On 14 June 2018 at 21:36, Auger Eric <address@hidden> wrote:
> Hi Peter,
>
> On 06/04/2018 05:29 PM, Peter Maydell wrote:
>> Implement the missing registers for the TZ MPC.
>>
>> Signed-off-by: Peter Maydell <address@hidden>

>> +    case A_INT_CLEAR:
>> +        if (value & R_INT_CLEAR_IRQ_MASK) {
>> +            s->int_stat = 0;
>> +            tz_mpc_irq_update(s);
> don't you need to clear the info regs. spec says:
> the [info] register retains its value until mpc_irq is cleared.

The full sentence is "Subsequent security violating transfers
remain blocked, that is, not captured in this register
and the register retains its value until mpc_irq is cleared."
I interpret "until mpc_irq is cleared" as applying to the
entire thing, ie mpc_irq being cleared is what allows a
subsequent transfer to be captured in this register.
(Hardware actively clearing itself to zero is unlikely,
because that costs extra gates which designers don't tend
to do unless there's a reason for it.)

>From a guest point of view (which is kind of the pov the
docs are written from), the guest can't rely on the register
value once mpc_irq is cleared (because another transaction
might come along and cause the value to be overwritten).

thanks
-- PMM