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

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Date:   Fri, 3 Jul 2020 18:54:15 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Al Viro <viro@...iv.linux.org.uk>
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 3, 2020 at 5:50 PM Al Viro <viro@...iv.linux.org.uk> wrote:
>
>     How could prefetcht0 possibly
> raise an exception?  Intel manual says that the only exception is #UD if
> LOCK PREFETCHT0 is encountered; not here, obviously.  AMD manual simply
> says "no exceptions".  Confused...

Several CPU bugs in this area. I think they may all have been AMD.

But we've definitely had "prefetch causes page faults" errata.

Google for it. One pdf (AMD errata) I found had this:

 "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."

So sadly the architecture manuals do not reflect reality.

That said, software prefetch instructions very seldom actually work.
They are only useful if you have one _very_ specific load and run one
one _very_ specific micrcoarchiecture.

Ir's almost always a mistake to have them in the first place.

            Linus