Re: [Qemu-devel] [PATCH] Fix ast2500 protection register emulation

[Peter Maydell]

On 20 February 2018 at 13:26, Hugo Landau <address@hidden> wrote:
> Some register blocks of the ast2500 are protected by protection key
> registers which require the right magic value to be written to those
> registers to allow those registers to be mutated.
>
> Register manuals indicate that writing the correct magic value to these
> registers should cause subsequent reads from those values to return 1,
> and writing any other value should cause subsequent reads to return 0.
>
> Previously, qemu implemented these registers incorrectly: the registers
> were handled as simple memory, meaning that writing some value x to a
> protection key register would result in subsequent reads from that
> register returning the same value x. The protection was implemented by
> ensuring that the current value of that register equaled the magic
> value.
>
> This modifies qemu to have the correct behaviour: attempts to write to a
> ast2500 protection register results in a transition to 1 or 0 depending
> on whether the written value is the correct magic. The protection logic
> is updated to ensure that the value of the register is nonzero.
>
> This bug caused deadlocks with u-boot HEAD: when u-boot is done with a
> protectable register block, it attempts to lock it by writing the
> bitwise inverse of the correct magic value, and then spinning forever
> until the register reads as zero. Since qemu implemented writes to these
> registers as ordinary memory writes, writing the inverse of the magic
> value resulted in subsequent reads returning that value, leading to
> u-boot spinning forever.
>
> Signed-off-by: Hugo Landau <address@hidden>

> -    if (addr != R_PROT && s->regs[R_PROT] != PROT_KEY_UNLOCK) {
> +    if (addr == R_PROT) {
> +      s->regs[addr] = (data == PROT_KEY_UNLOCK) ? 1 : 0;
> +      return;
> +    }

Applied to target-arm.next, thanks. I fixed up the incorrect indentation
in this part which checkpatch complains about.

-- PMM