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    SubjectRe: [patch 13/17] Immediate Values - x86 Optimization (updated)
    * H. Peter Anvin (hpa@zytor.com) wrote:
    > Mathieu Desnoyers wrote:
    >> Hrm, since even the nmi-safe version supports REX-prefixed instructions,
    >> there is no need for an =q constraint on single-byte immediate values
    >> anymore. (thanks to your "discard" section used in the nmi-safe version)
    >
    > WRONG!
    >
    > Using =r for single-byte values is incorrect for 32-bit code -- that would
    > permit %spl, %bpl, %sil, %dil which are illegal in 32-bit mode. That is not
    > the incorrect bit, it's the description that is confused.
    >
    > -hpa

    Ah, right. I remembered there was something that made us use =q, but
    could not remember what. I'll describe it correctly.

    Therefore, this updated patch is bogus. The original one was ok, given
    that we change the header to :


    Immediate Values - x86 Optimization

    x86 optimization of the immediate values which uses a movl with code patching
    to set/unset the value used to populate the register used as variable source.

    Note : a movb needs to get its value froma =q constraint.

    Quoting "H. Peter Anvin" <hpa@zytor.com>

    Using =r for single-byte values is incorrect for 32-bit code -- that would
    permit %spl, %bpl, %sil, %dil which are illegal in 32-bit mode.

    Changelog:
    - Use text_poke_early with cr0 WP save/restore to patch the bypass. We are doing
    non atomic writes to a code region only touched by us (nobody can execute it
    since we are protected by the imv_mutex).
    - Put imv_set and _imv_set in the architecture independent header.
    - Use $0 instead of %2 with (0) operand.
    - Add x86_64 support, ready for i386+x86_64 -> x86 merge.
    - Use asm-x86/asm.h.
    - Bugfix : 8 bytes 64 bits immediate value was declared as "4 bytes" in the
    immediate structure.
    - Change the immediate.c update code to support variable length opcodes.
    - Vastly simplified, using a busy looping IPI with interrupts disabled.
    Does not protect against NMI nor MCE.
    - Pack the __imv section. Use smallest types required for size (char).
    - Use imv_* instead of immediate_*.


    Mathieu

    --
    Mathieu Desnoyers
    Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
    OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68


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