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Subject[PATCH v4 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.

store_cpu_caps() basically stores the original CPU reported
values and not the OS modified values. This will avoid giving a false
warning even when 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.

Signed-off-by: Ashok Raj <ashok.raj@intel.com>
Fixes: 1008c52c09dc ("x86/CPU: Add a microcode loader callback")
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>
Cc: Ingo Molnar <mingo@kernel.org>
---
Changes since V3:
- Boris
- Change function from microcode_store_cpu_caps -> store_cpu_caps
- Split comments in store_cpu_caps().

- Dave Hansen
- Change parameters names to something meaninful.
- Cleaned up some commit log.

Changes since V2:

- Boris
- 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 | 39 ++++++++++++++++++++--------
arch/x86/kernel/cpu/microcode/core.c | 7 +++++
3 files changed, 36 insertions(+), 11 deletions(-)

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index f256a4ddd25d..a77dee6a2bf2 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -698,6 +698,7 @@ bool xen_set_default_idle(void);

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

enum l1tf_mitigations {
L1TF_MITIGATION_OFF,
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 0f5a173d0871..f5c6feed6c26 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -2297,6 +2297,29 @@ void cpu_init_secondary(void)
#endif

#ifdef CONFIG_MICROCODE_LATE_LOADING
+/**
+ * store_cpu_caps() - Store a snapshot of CPU capabilities
+ *
+ * Returns: None
+ */
+void store_cpu_caps(struct cpuinfo_x86 *curr_info)
+{
+ /* Reload CPUID max function as it might've changed. */
+ curr_info->cpuid_level = cpuid_eax(0);
+
+ /*
+ * Copy all capability leafs to pick up the synthetic ones
+ */
+ memcpy(&curr_info->x86_capability, &boot_cpu_data.x86_capability,
+ sizeof(curr_info->x86_capability));
+
+ /*
+ * Capabilities copied from BSP will get overwritten
+ * with the snapshot below
+ */
+ get_cpu_cap(curr_info);
+}
+
/**
* microcode_check() - Check if any CPU capabilities changed after an update.
* @prev_info: CPU capabilities stored before an update.
@@ -2309,22 +2332,16 @@ void cpu_init_secondary(void)
*/
void microcode_check(struct cpuinfo_x86 *prev_info)
{
- perf_check_microcode();
+ struct cpuinfo_x86 curr_info;

- /* Reload CPUID max function as it might've changed. */
- prev_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().
+ * Get a snapshot of CPU capabilities
*/
- memcpy(&prev_info->x86_capability, &boot_cpu_data.x86_capability,
- sizeof(prev_info->x86_capability));
-
- get_cpu_cap(prev_info);
+ store_cpu_caps(&curr_info);

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

diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
index e39d83be794b..bb943a91a364 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.
+ */
+ store_cpu_caps(&prev_info);
+
ret = stop_machine_cpuslocked(__reload_late, NULL, cpu_online_mask);
if (ret == 0)
microcode_check(&prev_info);
--
2.34.1
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