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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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