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    Subject[PATCH v2 1/2] x86: Enable fast strings on Intel if BIOS hasn't already
    Date
    Intel SDM volume 3A, 8.4.2 says:

    Software can disable fast-string operation by clearing the
    fast-string-enable bit (bit 0) of IA32_MISC_ENABLE MSR.
    However, Intel recomments that system software always enable
    fast-string operation.

    The Intel DQ67SW board (with latest BIOS) disables fast string
    operations if TXT is enabled. A Lenovo X220 disables it regardless
    of TXT setting. I doubt I'm the only person with a dumb BIOS like
    this.

    Signed-off-by: Andy Lutomirski <luto@mit.edu>
    ---
    arch/x86/kernel/cpu/intel.c | 25 ++++++++++++++++++++++---
    1 files changed, 22 insertions(+), 3 deletions(-)

    diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
    index ed6086e..c80ab41 100644
    --- a/arch/x86/kernel/cpu/intel.c
    +++ b/arch/x86/kernel/cpu/intel.c
    @@ -30,6 +30,7 @@
    static void __cpuinit early_init_intel(struct cpuinfo_x86 *c)
    {
    u64 misc_enable;
    + bool allow_fast_string = true;

    /* Unmask CPUID levels if masked: */
    if (c->x86 > 6 || (c->x86 == 6 && c->x86_model >= 0xd)) {
    @@ -118,8 +119,9 @@ static void __cpuinit early_init_intel(struct cpuinfo_x86 *c)
    * (model 2) with the same problem.
    */
    if (c->x86 == 15) {
    - rdmsrl(MSR_IA32_MISC_ENABLE, misc_enable);
    + allow_fast_string = false;

    + rdmsrl(MSR_IA32_MISC_ENABLE, misc_enable);
    if (misc_enable & MSR_IA32_MISC_ENABLE_FAST_STRING) {
    printk(KERN_INFO "kmemcheck: Disabling fast string operations\n");

    @@ -130,11 +132,28 @@ static void __cpuinit early_init_intel(struct cpuinfo_x86 *c)
    #endif

    /*
    - * If fast string is not enabled in IA32_MISC_ENABLE for any reason,
    - * clear the fast string and enhanced fast string CPU capabilities.
    + * If BIOS didn't enable fast string operation, try to enable
    + * it ourselves. If that fails, then clear the fast string
    + * and enhanced fast string CPU capabilities.
    */
    if (c->x86 > 6 || (c->x86 == 6 && c->x86_model >= 0xd)) {
    rdmsrl(MSR_IA32_MISC_ENABLE, misc_enable);
    +
    + if (allow_fast_string &&
    + !(misc_enable & MSR_IA32_MISC_ENABLE_FAST_STRING)) {
    + misc_enable |= MSR_IA32_MISC_ENABLE_FAST_STRING;
    + wrmsr_safe(MSR_IA32_MISC_ENABLE, (u32)misc_enable,
    + (u32)(misc_enable >> 32));
    +
    + /* Re-read to make sure it stuck. */
    + rdmsrl(MSR_IA32_MISC_ENABLE, misc_enable);
    +
    + if (misc_enable & MSR_IA32_MISC_ENABLE_FAST_STRING)
    + printk(KERN_WARNING FW_WARN "CPU #%d: "
    + "IA32_MISC_ENABLE.FAST_STRING_ENABLE "
    + "was not set", c->cpu_index);
    + }
    +
    if (!(misc_enable & MSR_IA32_MISC_ENABLE_FAST_STRING)) {
    printk(KERN_INFO "Disabled fast string operations\n");
    setup_clear_cpu_cap(X86_FEATURE_REP_GOOD);
    --
    1.7.6


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