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Date:   Fri, 17 Jun 2022 12:51:15 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Christophe Leroy' <christophe.leroy@...roup.eu>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
CC:     Matthew Wilcox <willy@...radead.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Linux-MM <linux-mm@...ck.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Uladzislau Rezki <urezki@...il.com>,
        "Kees Cook" <keescook@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Joe Perches <joe@...ches.com>
Subject: RE: [PATCH] usercopy: use unsigned long instead of uintptr_t

From: Christophe Leroy
> Sent: 17 June 2022 12:06
> 
> Le 17/06/2022 à 09:58, Geert Uytterhoeven a écrit :
> >> But it then causes pointless problems when people can't really rely on
> >> more than 32 bits for atomic bit operations, and on 64-bit
> >> architectures we unnecessarily use "long" and waste the upper bits.
> >
> > Well, atomic works up to native word size, i.e. long.
> >
> 
> powerpc64 has a pair of instructions to perform 128bits atomic
> operations : lqarx / stqcx.

As does x86-64 (and 32bit has a 64bit atomic compare+exchange).

Annoyingly the x86-64 doesn't have 128bit read/write register
pair instructions that would generate a 128bit PCIe TLP.
You can use AVX instructions to generate large TLP - but not
in the linux kernel - you want 1 big register.

Even the humble 68020 has a cas2 instruction that will do a
64bit atomic operation.
I did manage to use it once, but it is easier to disable interrupts.
I'm not sure how many SMP 68020 systems were ever built.
You'd need a matched pair of cpu (or extreme care) since it
tends to stack microcode data on mid-instruction faults.

	David

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