Re: [PATCH v3 13/18] target/arm: Update contiguous first-fault and no-fault loads

[Richard Henderson]

On 4/27/20 11:38 AM, Peter Maydell wrote:
> I would suggest something like:
> 
> +     * From this point on, all memory operations are MemSingleNF.
> +     *
> +     * Per the MemSingleNF pseudocode, a no-fault load from Device memory
> +     * must not actually hit the bus -- it returns (UNKNOWN, FAULT) instead.
> +     *
> +     * Unfortuately we do not have access to the memory attributes from the 
> PTE
> +     * to tell Device memory from Normal memory. So we make a mostly
> +     * correct check, and indicate (UNKNOWN, FAULT) for any MMIO.
> +     * This gives the right answer for the common cases of "Normal memory,
> +     * backed by host RAM" and "Device memory, backed by MMIO".
> +     * The architecture allows us to suppress an NF load and return
> +     * (UNKNOWN, FAULT) for any reason), so our behaviour (indicate
> +     * fault) for the corner case of "Normal memory, backed by MMIO" is
> +     * permitted. The case we get wrong is "Device memory, backed by
> +     * host RAM", which we should return (UNKNOWN, FAULT) for but do not.
> +     *
> +     * Similarly, CPU_BP breakpoints would raise exceptions, and so
> +     * return (UNKNOWN, FAULT).  For simplicity, we consider gdb and
> +     * architectural breakpoints the same.
> 
> assuming my understanding is correct...

Yep, thanks.  I'll merge this text.


r~