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Date: Thu, 29 Nov 2012 08:44:22 -0700
From: Peter Grehan <grehan@freebsd.org>
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To: Jilles Tjoelker <jilles@stack.nl>
Subject: Re: svn commit: r243667 - in projects/bhyve/sys/amd64/vmm: . intel
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> I don't see a check on the ModRM reg field, which indicates whether the
> 0x81 opcode is an ADD, OR, ADC, SBB, AND, SUB, XOR or CMP. So if one of
> the other seven happens, it will silently execute as an AND. I don't
> think it is useful to implement the other seven but they should be
> rejected explicitly (if the ModRM reg field is not 4).

  Will do, thanks.

> A hardware AND of any kind will update %rflags and compilers may take
> advantage of this to branch based on the result without the need for a
> separate instruction to test it. I suppose this is unlikely here (the
> AND serving only to clear one or more bits).

  Yeh... x86 sucks :(

later,

Peter.