Re: [Qemu-devel] qemu softmmu_template.h

["Blue Swirl" <blauwirbel@gmail.com>]

On 11/17/07, andrzej zaborowski <balrogg@gmail.com> 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.

Sparc is unaffected, unaligned accesses are forbidden.