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    Subject[PATCH v3 2/6] x86/microcode/core: Take a snapshot before and after applying microcode
    Date
    The kernel caches features about each CPU's features at boot in an
    x86_capability[] structure. The microcode update takes one snapshot and
    compares it with the saved copy at boot.

    However, the capabilities in the boot copy can be turned off as a result of
    certain command line parameters or configuration restrictions. This can
    cause a mismatch when comparing the values before and after the microcode
    update.

    microcode_check() is called after an update to report any previously
    cached CPUID bits might have changed due to the update.

    microcode_store_cpu_caps() basically stores the original CPU reported
    values and not the OS modified values. This will avoid giving a false
    warning even if no capabilities have changed.

    Ignore the capabilities recorded at boot. Take a new snapshot before the
    update and compare with a snapshot after the update to eliminate the false
    warning.

    Fixes: 1008c52c09dc ("x86/CPU: Add a microcode loader callback")
    Signed-off-by: Ashok Raj <ashok.raj@intel.com>
    Cc: LKML <linux-kernel@vger.kernel.org>
    Cc: x86 <x86@kernel.org>
    Cc: Tony Luck <tony.luck@intel.com>
    Cc: Dave Hansen <dave.hansen@intel.com>
    Cc: Alison Schofield <alison.schofield@intel.com>
    Cc: Reinette Chatre <reinette.chatre@intel.com>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Tom Lendacky <thomas.lendacky@amd.com>
    ---
    Changes since last post

    - Boris
    - Change function from copy_cpu_caps() -> store_cpu_caps()
    - Keep microcode_check() inside cpu/common.c and not bleed
    get_cpu_caps() outside of core code.

    - Thomas : Commit log changes.
    ---
    arch/x86/include/asm/processor.h | 1 +
    arch/x86/kernel/cpu/common.c | 31 +++++++++++++++++++++-------
    arch/x86/kernel/cpu/microcode/core.c | 7 +++++++
    3 files changed, 31 insertions(+), 8 deletions(-)

    diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
    index 387578049de0..ac2e67156b9b 100644
    --- a/arch/x86/include/asm/processor.h
    +++ b/arch/x86/include/asm/processor.h
    @@ -697,6 +697,7 @@ bool xen_set_default_idle(void);
    #endif

    void __noreturn stop_this_cpu(void *dummy);
    +void microcode_store_cpu_caps(struct cpuinfo_x86 *info);
    void microcode_check(struct cpuinfo_x86 *info);

    enum l1tf_mitigations {
    diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
    index b9c7529c920e..7c86c6fd07ae 100644
    --- a/arch/x86/kernel/cpu/common.c
    +++ b/arch/x86/kernel/cpu/common.c
    @@ -2297,28 +2297,43 @@ void cpu_init_secondary(void)
    #endif

    #ifdef CONFIG_MICROCODE_LATE_LOADING
    +
    +void microcode_store_cpu_caps(struct cpuinfo_x86 *info)
    +{
    + /* Reload CPUID max function as it might've changed. */
    + info->cpuid_level = cpuid_eax(0);
    +
    + /*
    + * Copy all capability leafs to pick up the synthetic ones so that
    + * memcmp() below doesn't fail on that. The ones coming from CPUID will
    + * get overwritten in get_cpu_cap().
    + */
    + memcpy(info->x86_capability, &boot_cpu_data.x86_capability,
    + sizeof(info->x86_capability));
    +
    + get_cpu_cap(info);
    +}
    +
    /*
    * The microcode loader calls this upon late microcode load to recheck features,
    * only when microcode has been updated. Caller holds microcode_mutex and CPU
    * hotplug lock.
    */
    -void microcode_check(struct cpuinfo_x86 *info)
    +void microcode_check(struct cpuinfo_x86 *orig)
    {
    - perf_check_microcode();
    + struct cpuinfo_x86 info;

    - /* Reload CPUID max function as it might've changed. */
    - info->cpuid_level = cpuid_eax(0);
    + perf_check_microcode();

    /*
    * Copy all capability leafs to pick up the synthetic ones so that
    * memcmp() below doesn't fail on that. The ones coming from CPUID will
    * get overwritten in get_cpu_cap().
    */
    - memcpy(&info->x86_capability, &boot_cpu_data.x86_capability, sizeof(info->x86_capability));
    -
    - get_cpu_cap(info);
    + microcode_store_cpu_caps(&info);

    - if (!memcmp(&info->x86_capability, &boot_cpu_data.x86_capability, sizeof(info->x86_capability)))
    + if (!memcmp(&info.x86_capability, &orig->x86_capability,
    + sizeof(info.x86_capability)))
    return;

    pr_warn("x86/CPU: CPU features have changed after loading microcode, but might not take effect.\n");
    diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
    index d86a4f910a6b..14d9031ed68a 100644
    --- a/arch/x86/kernel/cpu/microcode/core.c
    +++ b/arch/x86/kernel/cpu/microcode/core.c
    @@ -447,6 +447,13 @@ static int microcode_reload_late(void)
    atomic_set(&late_cpus_in, 0);
    atomic_set(&late_cpus_out, 0);

    + /*
    + * Take a snapshot before the microcode update, so we can compare
    + * them after the update is successful to check for any bits
    + * changed.
    + */
    + microcode_store_cpu_caps(&info);
    +
    ret = stop_machine_cpuslocked(__reload_late, NULL, cpu_online_mask);
    if (ret == 0)
    microcode_check(&info);
    --
    2.34.1
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