Re: [Qemu-devel] qemu softmmu_template.h

[andrzej zaborowski]

On 17/11/2007, J. Mayer <address@hidden> wrote:
>
> On Sat, 2007-11-17 at 11:44 +0100, andrzej zaborowski wrote:
> > On 17/11/2007, J. Mayer <address@hidden> wrote:
> > >
> > > On Sat, 2007-11-17 at 11:14 +0100, andrzej zaborowski wrote:
> > > > On 17/11/2007, J. Mayer <address@hidden> 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.
>
> One case that obviously can have nasty side effects is if doing
> unaligned IO accesses. Doing accesses from first byte to the last is
> very different than doing the access from the last to the first.

Hmm, right, I had not thought about IO accesses. I will watch for
reports of any breakage that may have any connection with this and
revert if there's any such report.

> What also can be very different is what is to happen when the
> instruction is to be restarted because of a page fault.
> I checked the PowerPC specification, and it appears that it allows
> splitted memory accesses to be done in any order. It also specifies that
> load and stores are restartable even if they have been partially
> executed (ie some registers or memory locations have already been
> changed), then this patch is likely not to break this target (but I did
> not check all specific implementations to see if some have specific
> requirements).
> This is to be checked for all other targets before such a patch can be
> applied, imho.

Yes, although in practice that means the workaround (not a proper
bugfix) would never be in qemu CVS and would be maintained in other
trees endlessly.