[PATCH 09/13] x86/microcode: Add a generic mechanism to declare support for minrev

[Ashok Raj]

Intel microcode adds some meta-data to report a minimum required revision
before this new microcode can be late-loaded. There are no generic mechanism
to declare support for all vendors.

Add generic support to microcode to declare support, so the tainting and
late-loading can be permitted in those architectures that support reporting
a minrev in some form.

Late loading has added support for

- New images declaring a required minimum base version before a late-load
  is performed.
- Improved NMI handling during update to avoid sibling threads taking NMI's
  while primary is still not complete with the microcode update.

With these changes, late-loading can be re-enabled. Tainting only happens
on architectures that don't support minimum required version reporting.

Signed-off-by: Ashok Raj <ashok.raj@xxxxxxxxx>
---
 arch/x86/include/asm/microcode.h      |  1 +
 arch/x86/kernel/cpu/microcode/core.c  | 14 +++++++++++---
 arch/x86/kernel/cpu/microcode/intel.c |  6 ++++++
 arch/x86/Kconfig                      |  7 ++++---
 4 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/microcode.h b/arch/x86/include/asm/microcode.h
index 401213fb2e4a..0c0bbc26560f 100644
--- a/arch/x86/include/asm/microcode.h
+++ b/arch/x86/include/asm/microcode.h
@@ -59,6 +59,7 @@ enum ucode_state {
 };
 
 struct microcode_ops {
+	int (*check_minrev) (void);
 	enum ucode_state (*request_microcode_fw) (int cpu, struct device *,
 						  bool late_loading);
 
diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
index 7a8fcb914b6a..46e9c2d8fae0 100644
--- a/arch/x86/kernel/cpu/microcode/core.c
+++ b/arch/x86/kernel/cpu/microcode/core.c
@@ -606,6 +606,7 @@ static ssize_t reload_store(struct device *dev,
 	enum ucode_state tmp_ret = UCODE_OK;
 	int bsp = boot_cpu_data.cpu_index;
 	unsigned long val;
+	int minrev;
 	ssize_t ret = 0;
 
 	ret = kstrtoul(buf, 0, &val);
@@ -621,8 +622,14 @@ static ssize_t reload_store(struct device *dev,
 	if (ret)
 		goto put;
 
-	pr_err("Attempting late microcode loading - it is dangerous and taints the kernel.\n");
-	pr_err("You should switch to early loading, if possible.\n");
+	if (microcode_ops->check_minrev())
+		minrev = microcode_ops->check_minrev();
+
+	if (!minrev) {
+		pr_err("Attempting late microcode loading - it is dangerous and taints the kernel.\n");
+		pr_err("You should switch to early loading, if possible.\n");
+	}
+
 	tmp_ret = microcode_ops->request_microcode_fw(bsp, &microcode_pdev->dev, true);
 	if (tmp_ret != UCODE_NEW)
 		goto put;
@@ -637,7 +644,8 @@ static ssize_t reload_store(struct device *dev,
 	if (ret == 0)
 		ret = size;
 
-	add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK);
+	if (!minrev)
+		add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK);
 
 	return ret;
 }
diff --git a/arch/x86/kernel/cpu/microcode/intel.c b/arch/x86/kernel/cpu/microcode/intel.c
index 46edce811c69..c8ee53fcf04d 100644
--- a/arch/x86/kernel/cpu/microcode/intel.c
+++ b/arch/x86/kernel/cpu/microcode/intel.c
@@ -950,7 +950,13 @@ static enum ucode_state request_microcode_fw(int cpu, struct device *device,
 	return ret;
 }
 
+static int intel_check_minrev(void)
+{
+	return 1;
+}
+
 static struct microcode_ops microcode_intel_ops = {
+	.check_minrev			  = intel_check_minrev,
 	.request_microcode_fw             = request_microcode_fw,
 	.collect_cpu_info                 = collect_cpu_info,
 	.apply_microcode                  = apply_microcode_intel,
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index f9920f1341c8..a01fce1092ce 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1336,15 +1336,16 @@ config MICROCODE_AMD
 	  processors will be enabled.
 
 config MICROCODE_LATE_LOADING
-	bool "Late microcode loading (DANGEROUS)"
-	default n
+	bool "Late microcode loading"
+	default y
 	depends on MICROCODE
 	help
 	  Loading microcode late, when the system is up and executing instructions
 	  is a tricky business and should be avoided if possible. Just the sequence
 	  of synchronizing all cores and SMT threads is one fragile dance which does
 	  not guarantee that cores might not softlock after the loading. Therefore,
-	  use this at your own risk. Late loading taints the kernel too.
+	  use this at your own risk. Late loading taints the kernel, if it
+	  doesn't support a minimum required base version before an update.
 
 config X86_MSR
 	tristate "/dev/cpu/*/msr - Model-specific register support"
-- 
2.34.1