* Andy Lutomirski <l...@amacapital.net> wrote:
> On Sep 14, 2015 11:00 PM, "Ingo Molnar" <mi...@kernel.org> wrote:
> >
> >
> > * Andy Lutomirski <l...@amacapital.net> wrote:
> >
> > > On Sep 14, 2015 1:15 AM, "Ingo Molnar" <mi...@kernel.org> wrote:
> > > >
> > > >
> > > > * Denys Vlasenko <dvlas...@redhat.com> wrote:
> > > >
> > > > > >> + /* INT80 syscall entrypoint can be used by
> > > > > >> + * 64-bit programs too, unlike
> > > > > >> SYSCALL/SYSENTER.
> > > > > >> + * Therefore it must preserve R12+
> > > > > >> + * (they are callee-saved registers in
> > > > > >> 64-bit C ABI).
> > > > > >> + *
> > > > > >> + * This was probably historically not
> > > > > >> intended,
> > > > > >> + * but R8..11 are clobbered (cleared to 0).
> > > > > >> + * IOW: they are the only registers which
> > > > > >> aren't
> > > > > >> + * preserved across INT80 syscall.
> > > > > >> + */
> > > > > >> + if (*r64 == 0 && num <= 11)
> > > > > >> + continue;
> > > > > >
> > > > > > Ugh. I'll change my big entry patchset to preserve these and maybe
> > > > > > to
> > > > > > preserve all of the 64-bit regs.
> > > > >
> > > > > If you do that, this won't change the ABI: we don't _promise_
> > > > > to save them. If we accidentally do, that means nothing.
> > > >
> > > > Argh, that's dangerous nonsense! You _still_ don't seem to understand
> > > > what the
> > > > Linux ABI means and how to change code that implements it...
> > >
> > > I think Denys might be taking about R8-R11 here. If we change them
> > > from clobbered to saved, that's probably fine. Certainly I have to
> > > save R12-R15 -- my v1 is just buggy there. I was too deep in
> > > __kernel_vsyscall when I wrote that code, and I wasn't thinking about
> > > the raw int $0x80 entry variant.
> > >
> > > I'd be rather surprised if anything broke if we started preserving
> > > R8-R11 instead of zeroing them.
> >
> > Well, read the statement:
> >
> > " If you do that, this won't change the ABI: we don't _promise_
> > to save them. If we accidentally do, that means nothing. "
> >
> > of _course_ it means everything: if we preserve R8-R11 and any app learns
> > to rely
> > on it, it becomes an ABI.
>
> Right, it changes the ABI in a way that we can't undo, but it probably
> doesn't break the old ABI.
it's unknown: user-space code might have (unknowingly) started relying on the
zeroing behavior. If that happened (and let's hope it didn't - but there's no
guarantee), then the zeroing behavior is an ABI too.
> Certainly for v2, I'll try to preserve the old behavior exactly. If
> we change it later to preserve all high regs, that'll be a separate
> patch.
Yeah, cleanly separating ABI-invariant patches from ABI-impacting ones (no
matter
how innocious the effect on the ABI looks) is a must.
Thanks,
Ingo
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