Re: [Qemu-devel] qemu softmmu_template.h

[Fabrice Bellard <fabrice@bellard.org>]

andrzej zaborowski wrote:
> On 17/11/2007, J. Mayer <l_indien@magic.fr> wrote:
>> On Sat, 2007-11-17 at 11:14 +0100, andrzej zaborowski wrote:
>>> On 17/11/2007, J. Mayer <l_indien@magic.fr> wrote:
>>>> On Sat, 2007-11-17 at 09:53 +0000, Andrzej Zaborowski wrote:
>>>>> CVSROOT:      /sources/qemu
>>>>> Module name:  qemu
>>>>> Changes by:   Andrzej Zaborowski <balrog>     07/11/17 09:53:42
>>>>>
>>>>> Modified files:
>>>>>       .              : softmmu_template.h
>>>>>
>>>>> Log message:
>>>>>       Check permissions for the last byte first in unaligned slow_st accesses (patch from TeLeMan).
>>>>>
>>>>> CVSWeb URLs:
>>>>> http://cvs.savannah.gnu.org/viewcvs/qemu/softmmu_template.h?cvsroot=qemu&r1=1.19&r2=1.20
>>>>>
>>>> Has it been checked that it's legal for all architectures and cannot
>>>> have any nasty side effect to do accesses in the reverse order ? Real
>>>> hardware do not ever seem to do this...
>>> For real hardware the store is a single operation.
>> For PowerPC, at least, only aligned stores are defined as atomic. It's
>> absolutely legal for an implementation to split all non-atomic accesses
>> into smaller aligned accesses. And I guess it is the same for all
>> architecture that can do unaligned accesses.
>>
>>> Logically it shouldn't have any side effects, but if it does then it
>>> would rather mean that other code for that architecture is (also)
>>> broken, I believe.
>>>
>>> I've only tested ARM, mips, x86 and x86_64 before committing, so
>>> please test. I figured that the patch won't get any comments on the
>>> mailing list if it isn't merged.
>> I don't think it's so easy to test because it may be very  hard to
>> trigger the cases that would have side effects, which are target
>> dependent. I then am very curious to know how you did check that there
>> is no problem with this patch....
> 
> Well, for ARM, x86 and x86_64 I only checked that unaligned accesses
> still work, i.e. that I haven't made an obvious typo. I haven't tested
> cross-page accesses with the access to the second page being invalid,
> I also don't know how the specifications for other architectures
> define the effect of such accesses, so maybe I shouldn't have
> committed this, but I assumed a common sense in the design of cpu
> archs, meaning that in the example given by TeLeMan the addition is
> not performed two times on some bytes.
> Regards

I agree with this patch is the sense that the previous behaviour was
clearly incorrect.

Now this patch relies on the fact that tlb_fill() does not remove the
previous page from the TLB cache which is an important "hidden" constraint.

Fabrice.