linux-kernel - Re: objtool clac/stac handling change..

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Message-ID: <20200704023009.GA2786714@ZenIV.linux.org.uk>
Date:   Sat, 4 Jul 2020 03:30:09 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Christophe Leroy <christophe.leroy@....fr>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: objtool clac/stac handling change..

On Fri, Jul 03, 2020 at 06:54:15PM -0700, Linus Torvalds wrote:

>  "Software Prefetches May Report A Page Fault
> 
>   Description Software prefetch instructions are defined to ignore
> page faults. Under highly specific and detailed internal
> circumstances, a prefetch instruction may report a page fault if both
> of the following conditions are true:
> 
>    • The target address of the prefetch would cause a page fault if
> the address was accessed by an actual memory load or store instruction
> under the current privilege mode;
> 
>    • The prefetch instruction is followed in execution-order by an
> actual or speculative byte-sized memory access of the same
> modify-intent to the same address. PREFETCH and PREFETCHNTA/0/1/2 have
> the same modify-intent as a memory load access.
> 
>   PREFETCHW has the same modify-intent as a memory store access. The
> page fault exception error code bits for the faulting prefetch will be
> identical to that for a bytesized memory access of the same-modify
> intent to the same address. Note that some misaligned accesses can be
> broken up by the processor into multiple accesses where at least one
> of the accesses is a byte-sized access. If the target address of the
> subsequent memory access of the same modify-intent is aligned and not
> byte-sized, this errata does not occur and no workaround is needed.
> 
>   Potential Effect on System An unexpected page fault may occur
> infrequently on a prefetch instruction."

Lovely...  So basically this is the rare place where we might use those
insns on userland addresses?