Re: [PATCH v2 05/11] hw/intc/gic: use MxTxAttrs to divine accessing CPU

["Alex Bennée" <alex.bennee@linaro.org>]

Peter Maydell <peter.maydell@linaro.org> writes:

> On Mon, 26 Sept 2022 at 14:39, Alex Bennée <alex.bennee@linaro.org> wrote:
>>
>> Now that MxTxAttrs encodes a CPU we should use that to figure it out.
>> This solves edge cases like accessing via gdbstub or qtest.
>>
>> Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
>> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
>> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/124
>>
>> ---
>> v2
>>   - update for new field
>>   - bool asserts
>> ---
>>  hw/intc/arm_gic.c | 39 ++++++++++++++++++++++-----------------
>>  1 file changed, 22 insertions(+), 17 deletions(-)
>>
>> diff --git a/hw/intc/arm_gic.c b/hw/intc/arm_gic.c
>> index 492b2421ab..d907df3884 100644
>> --- a/hw/intc/arm_gic.c
>> +++ b/hw/intc/arm_gic.c
>> @@ -56,17 +56,22 @@ static const uint8_t gic_id_gicv2[] = {
>>      0x04, 0x00, 0x00, 0x00, 0x90, 0xb4, 0x2b, 0x00, 0x0d, 0xf0, 0x05, 0xb1
>>  };
>>
>> -static inline int gic_get_current_cpu(GICState *s)
>> +static inline int gic_get_current_cpu(GICState *s, MemTxAttrs attrs)
>>  {
>> -    if (!qtest_enabled() && s->num_cpu > 1) {
>> -        return current_cpu->cpu_index;
>> -    }
>> -    return 0;
>> +    /*
>> +     * Something other than a CPU accessing the GIC would be a bug as
>> +     * would a CPU index higher than the GICState expects to be
>> +     * handling
>> +     */
>> +    g_assert(attrs.requester_type == MEMTXATTRS_CPU);
>> +    g_assert(attrs.requester_id < s->num_cpu);
>
> Would it be a QEMU bug, or a guest code bug ? If it's possible
> for the guest to mis-program a DMA controller to do a read that
> goes through this function, we shouldn't assert. (Whether that
> can happen will depend on how the board/SoC code puts together
> the MemoryRegion hierarchy, I think.)

Most likely a QEMU bug - how would a DMA master even access the GIC?

>
> thanks
> -- PMM


-- 
Alex Bennée