Re: [PATCH 4/8] s390/entry: workaround llvm's IAS limitations

[Jonas Paulsson]

I will try to get a patch for clang ready soon... /Jonas

On 2022-05-16 12:19 em, Heiko Carstens wrote:
> On Mon, May 16, 2022 at 11:07:43AM +0200, Alexander Gordeev wrote:
> > > diff --git a/arch/s390/kernel/entry.S b/arch/s390/kernel/entry.S
> > > index a6b45eaa3450..f2f30bfba1e9 100644
> > > --- a/arch/s390/kernel/entry.S
> > > +++ b/arch/s390/kernel/entry.S
> > > @@ -172,9 +172,19 @@ _LPP_OFFSET	= __LC_LPP
> > >   	lgr	%r14,\reg
> > >   	larl	%r13,\start
> > >   	slgr	%r14,%r13
> > > -	lghi	%r13,\end - \start
> > > -	clgr	%r14,%r13
> > > +#ifdef CONFIG_AS_IS_LLVM
> > > +	clgfrl	%r14,.Lrange_size\@
> > > +#else
> > > +	clgfi	%r14,\end - \start
> > > +#endif
> > >   	jhe	\outside_label
> > > +#ifdef CONFIG_CC_IS_CLANG
> > > +	.section .rodata, "a"
> > > +	.align 4
> > > +.Lrange_size\@:
> > > +	.long	\end - \start
> > Isn't the machine check handler refers to this memory before checking
> > unrecoverable storage errors (with CHKSTG macro) as result of this change?
> Yes, indeed. However implementing this without another register will
> be quite of a challenge. So what I would prefer in any case: just
> assume that this minimal set of memory accesses work. Actually I'd
> seriously like to go a bit further, and even move the checks for
> storage errors back to C for two reasons:
>
> - this would make the machine check handler entry code easier again
> - it would also allow to enter the machine check handler with DAT on
>
> After all we rely anyway on the fact that at least the local lowcore +
> the page(s) which contain text are still accessible. Assuming that a
> couple of page tables also work won't make this much worse, but the
> code much easier.
>
> So I'd suggest: leave this code as is, and at some later point move
> "rework" the early machine check handler code.
>
> What do you think?