Re: [Qemu-devel] [PATCH] target-ppc: Multiple/String Word alignment exception

[Alexander Graf <agraf@suse.de>]

On 31.03.16 09:06, Laurent Vivier wrote:
> 
> 
> On 31/03/2016 08:54, Alexander Graf wrote:
>>
>>
>> On 31.03.16 01:29, David Gibson wrote:
>>> On Wed, 30 Mar 2016 19:13:00 +0200
>>> Laurent Vivier <lvivier@redhat.com> wrote:
>>>
>>>> If the processor is in little-endian mode, an alignment interrupt must
>>>> occur for the following instructions: lmw, stmw, lswi, lswx, stswi or stswx.
>>>>
>>>> This is what happens with KVM, so change TCG to do the same.
>>>>
>>>> As the instruction can be emulated by the kernel, enable the change
>>>> only in softmmu mode.
>>>>
>>>> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
>>>
>>> I guess this makes sense given the existing hardware behaviour, even
>>> though it seems a bit perverse to me to make the emulator strictly less
>>> functional.
>>>
>>> Alex, what do you think?
>>
>> In general we only implement strict checks if it breaks guests not to
>> have them. Are you aware of any such case?
> 
> No, it does not break anything. The idea was to have the same behavior
> with TCG as with a real CPU (or kvm). But if it is not the rule, we can
> drop this patch.

I guess if you really care about same behavior, we'd need to have risu
ported to ppc. However, that's a huge can of worms. Once we start that,
we'd have to verify risu against every single CPU type we support
because they all interpret certain corner cases differently.

That's basically what David was trying to say with POWER9. How do you
know that POWER9 still requires strong alignment checks for indexed LE
instructions? If it doesn't, we'd have to add a case in TCG to not the
the checks again. These multiply very quickly :).


Alex