Re: [Qemu-devel] qemu softmmu_template.h

["J. Mayer" <l_indien@magic.fr>]

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....
I have to admit I did not notice this patch, or I would have commented
it before (my fault).

-- 
J. Mayer <l_indien@magic.fr>
Never organized