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    Subject[PATCH v7 05/14] x86/alternative: support not-feature
    Date
    Add support for alternative patching for the case a feature is not
    present on the current cpu.

    For users of ALTERNATIVE() and friends an inverted feature is specified
    by applying the ALT_NOT() macro to it, e.g.:

    ALTERNATIVE(old, new, ALT_NOT(feature))

    Signed-off-by: Juergen Gross <jgross@suse.com>
    ---
    V5:
    - split off from next patch
    - reworked to use flag byte (Boris Petkov)
    V6:
    - rework again to not use flag byte (Boris Petkov)
    V7:
    - minor tweaks: move defines up, comment wording (Boris Petkov)
    ---
    arch/x86/include/asm/alternative.h | 3 +++
    arch/x86/kernel/alternative.c | 20 +++++++++++++++-----
    2 files changed, 18 insertions(+), 5 deletions(-)

    diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
    index 53f295f41c34..649e56f70889 100644
    --- a/arch/x86/include/asm/alternative.h
    +++ b/arch/x86/include/asm/alternative.h
    @@ -6,6 +6,9 @@
    #include <linux/stringify.h>
    #include <asm/asm.h>

    +#define ALTINSTR_FLAG_INV (1 << 15)
    +#define ALT_NOT(feat) ((feat) | ALTINSTR_FLAG_INV)
    +
    #ifndef __ASSEMBLY__

    #include <linux/stddef.h>
    diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
    index 8d778e46725d..133b549dc091 100644
    --- a/arch/x86/kernel/alternative.c
    +++ b/arch/x86/kernel/alternative.c
    @@ -388,21 +388,31 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
    */
    for (a = start; a < end; a++) {
    int insn_buff_sz = 0;
    + /* Mask away "NOT" flag bit for feature to test. */
    + u16 feature = a->cpuid & ~ALTINSTR_FLAG_INV;

    instr = (u8 *)&a->instr_offset + a->instr_offset;
    replacement = (u8 *)&a->repl_offset + a->repl_offset;
    BUG_ON(a->instrlen > sizeof(insn_buff));
    - BUG_ON(a->cpuid >= (NCAPINTS + NBUGINTS) * 32);
    - if (!boot_cpu_has(a->cpuid)) {
    + BUG_ON(feature >= (NCAPINTS + NBUGINTS) * 32);
    +
    + /*
    + * Patch if either:
    + * - feature is present
    + * - feature not present but ALTINSTR_FLAG_INV is set to mean,
    + * patch if feature is *NOT* present.
    + */
    + if (!boot_cpu_has(feature) == !(a->cpuid & ALTINSTR_FLAG_INV)) {
    if (a->padlen > 1)
    optimize_nops(a, instr);

    continue;
    }

    - DPRINTK("feat: %d*32+%d, old: (%pS (%px) len: %d), repl: (%px, len: %d), pad: %d",
    - a->cpuid >> 5,
    - a->cpuid & 0x1f,
    + DPRINTK("feat: %s%d*32+%d, old: (%pS (%px) len: %d), repl: (%px, len: %d), pad: %d",
    + (a->cpuid & ALTINSTR_FLAG_INV) ? "!" : "",
    + feature >> 5,
    + feature & 0x1f,
    instr, instr, a->instrlen,
    replacement, a->replacementlen, a->padlen);

    --
    2.26.2
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