Re: Silence compiler warning in arch/x86/kvm/emulate.c

From	Valdis.Kletnieks@vt.edu
Newsgroups	linux.kernel
Subject	Re: Silence compiler warning in arch/x86/kvm/emulate.c
Date	2016-02-20 01:40 +0100
Message-ID	<r43Kx-5aw-3@gated-at.bofh.it> (permalink)
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On Fri, 19 Feb 2016 18:56:05 +0100, Paolo Bonzini said:
> On 19/02/2016 18:54, Valdis.Kletnieks@vt.edu wrote:
> > But under what conditions is 'la' a 64-bit quantity that has any
> > bits set in the high 32 bits (meaning it's a pointer to something
> > over the 4G line) - but it's still valid to smash those bits?
>
> That can happen for example if there is a non-zero segment base.  Then
> the linear address wraps at 4G.

Gaah.

Obviously, the concept that software could actually depend on a segment base
pointing at the 3G line (or whatever) to wrap around and be used to address
memory down in the first gig of RAM was too bizarre for my brain to visualize.
:)

The IBM S/360 with 24 bit addresses and S/370 with 24 or 31 bit addresses would
allow some instructions (most famously MVCL Move Character Long) to start
operating at the high end of memory and wrap around to the beginning.  The
system documentation was pretty clear that although this *worked*, it was
probably not what you actually wanted to do.... :)

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Re: Silence compiler warning in  arch/x86/kvm/emulate.c Aurelien Jarno <aurelien@aurel32.net> - 2016-02-19 13:00 +0100
  Re: Silence compiler warning in arch/x86/kvm/emulate.c Paolo Bonzini <pbonzini@redhat.com> - 2016-02-19 13:10 +0100
    Re: Silence compiler warning in arch/x86/kvm/emulate.c Aurelien Jarno <aurelien@aurel32.net> - 2016-02-19 18:10 +0100
  Re: Silence compiler warning in  arch/x86/kvm/emulate.c Aurelien Jarno <aurelien@aurel32.net> - 2016-02-19 17:50 +0100
    Re: Silence compiler warning in arch/x86/kvm/emulate.c Valdis.Kletnieks@vt.edu - 2016-02-19 19:00 +0100
      Re: Silence compiler warning in arch/x86/kvm/emulate.c Paolo Bonzini <pbonzini@redhat.com> - 2016-02-19 19:00 +0100
        Re: Silence compiler warning in arch/x86/kvm/emulate.c Valdis.Kletnieks@vt.edu - 2016-02-20 01:40 +0100

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