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

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Message-ID: <YoIxdMNJjt9rxoeZ@tuxmaker.boeblingen.de.ibm.com>
Date:   Mon, 16 May 2022 13:11:48 +0200
From:   Alexander Gordeev <agordeev@...ux.ibm.com>
To:     Heiko Carstens <hca@...ux.ibm.com>
Cc:     Nathan Chancellor <nathan@...nel.org>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Jonas Paulsson <paulsson@...ux.vnet.ibm.com>,
        Ulrich Weigand <ulrich.weigand@...ibm.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Alexander Egorenkov <egorenar@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Andreas Krebbel <krebbel@...ux.ibm.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org
Subject: Re: [PATCH 4/8] s390/entry: workaround llvm's IAS limitations

On Mon, May 16, 2022 at 12:19:45PM +0200, Heiko Carstens wrote:
> On Mon, May 16, 2022 at 11:07:43AM +0200, Alexander Gordeev wrote:
> > 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?

Sounds very reasonable. Please, find my:

Acked-by: Alexander Gordeev <agordeev@...ux.ibm.com>


Also, how such a follow-up looks to you?

	lgr	%r14,\reg
#ifdef CONFIG_AS_IS_LLVM
	larl	%r13,\start
	slgr	%r14,%r13
	clgfrl	%r14,.Lrange_size\@
#else
	slgfi	%r14,\start
	clgfi	%r14,\end - \start
#endif