linux-kernel - Re: [PATCH] Alpha: Emulate unaligned LDx_L/STx

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Message-ID: <alpine.DEB.2.21.2502252030380.65342@angie.orcam.me.uk>
Date: Tue, 25 Feb 2025 20:44:20 +0000 (GMT)
From: "Maciej W. Rozycki" <macro@...am.me.uk>
To: Richard Henderson <richard.henderson@...aro.org>, 
    Ivan Kokshaysky <ink@...een.parts>, Matt Turner <mattst88@...il.com>
cc: Arnd Bergmann <arnd@...db.de>, 
    John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>, 
    Magnus Lindholm <linmag7@...il.com>, 
    "Paul E. McKenney" <paulmck@...nel.org>, 
    Linus Torvalds <torvalds@...ux-foundation.org>, 
    Al Viro <viro@...iv.linux.org.uk>, linux-alpha@...r.kernel.org, 
    linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Alpha: Emulate unaligned LDx_L/STx_C for data
 consistency

On Wed, 19 Feb 2025, Maciej W. Rozycki wrote:

> Interestingly enough no kernel mode traps have triggered with a kernel 
> built with GCC 12 (and with most user traps coming from GCC verification):
> 
> kernel unaligned acc	: 0 (pc=0,va=0)
> user unaligned acc	: 1766720 (pc=20000053064,va=120020189)
> 
> but with GCC 15 a small quantity happened (even before I ran GCC testing):
> 
> kernel unaligned acc    : 78 (pc=fffffc0000ad5194,va=fffffc0002db5784)
> user unaligned acc      : 883452 (pc=20000053064,va=120020189)
> 
> It seems a compiler regression worth checking -- the trap recorded was in 
> `icmp6_dst_alloc' with a pair of quadword writes to `rt->rt6i_dst.addr', 
> which however by its type (`struct in6_addr') is only longword-aligned and 
> indeed starts at offset 148 from the outermost struct.  I have a sneaking 
> suspicion one of my earlier GCC changes might be at fault.  At least I now 
> have a test case to experiment with.

 FYI my suspicion wasn't wrong, I have submitted a compiler fix now[1].  

 My plan has been to complete the GCC side first as it's more urgent given 
its annual only release cycle model targetting April/May, whereas I think 
the Linux side can slip a release or two in our roughly bi-monthly cycle.  
I'm going to schedule my time accordinly and with my upcoming holiday also 
in the picture I may not be able to post v2 of this proposal until around 
end of March the earliest.

References:

[1] "Alpha: Fix base block alignment calculation regression", 
    <https://inbox.sourceware.org/gcc-patches/alpine.DEB.2.21.2502251934260.65342@angie.orcam.me.uk/>

  Maciej