Re: [PATCH for-6.2 17/43] accel/tcg: Report unaligned atomics for user-only

[Philippe Mathieu-Daudé]

On 7/29/21 5:02 PM, Peter Maydell wrote:
> On Thu, 29 Jul 2021 at 02:09, Richard Henderson
> <richard.henderson@linaro.org> wrote:
>>
>> Use the newly exposed do_unaligned_access hook from atomic_mmu_lookup,
>> which has access to complete alignment info from the TCGMemOpIdx arg.
>>
>> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
>> ---
>>  accel/tcg/user-exec.c | 23 ++++++++++++++++++++++-
>>  1 file changed, 22 insertions(+), 1 deletion(-)
>>
>> diff --git a/accel/tcg/user-exec.c b/accel/tcg/user-exec.c
>> index 90d1a2d327..dd77e90789 100644
>> --- a/accel/tcg/user-exec.c
>> +++ b/accel/tcg/user-exec.c
>> @@ -852,6 +852,16 @@ int cpu_signal_handler(int host_signum, void *pinfo,
>>
>>  /* The softmmu versions of these helpers are in cputlb.c.  */
>>
>> +static void cpu_unaligned_access(CPUState *cpu, vaddr addr,
>> +                                 MMUAccessType access_type,
>> +                                 int mmu_idx, uintptr_t ra)
>> +{
>> +    CPUClass *cc = CPU_GET_CLASS(cpu);
>> +
>> +    cc->tcg_ops->do_unaligned_access(cpu, addr, access_type, mmu_idx, ra);
>> +    g_assert_not_reached();
>> +}
> 
> The softmmu version doesn't g_assert_not_reached(), I think
> perhaps with the intent that a CPU implementation could
> in some cases return without raising an exception to
> mean "continue with the unaligned access". We should decide
> whether we want the API to permit that, or else consistently
> have both softmmu and useronly versions be marked noreturn
> and with an assert, and we should document whichever we choose.

Agreed. I'd rather use noreturn, which exposed these bugs:
- "target/xtensa: clean up unaligned access"
https://lists.gnu.org/archive/html/qemu-devel/2021-05/msg05890.html
- "target/nios2: Mark raise_exception() as noreturn"
https://lists.gnu.org/archive/html/qemu-devel/2021-07/msg07001.html