[PATCH v4 2/6] x86/microcode/core: Take a snapshot before and after applying microcode

[Ashok Raj]

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@xxxxxxxxx>
Fixes: 1008c52c09dc ("x86/CPU: Add a microcode loader callback")
Cc: LKML <linux-kernel@xxxxxxxxxxxxxxx>
Cc: x86 <x86@xxxxxxxxxx>
Cc: Tony Luck <tony.luck@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: Alison Schofield <alison.schofield@xxxxxxxxx>
Cc: Reinette Chatre <reinette.chatre@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
---
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