[PATCH v9 10/17] x86/fsgsbase/64: Enable FSGSBASE instructions in helper functions

[Chang S. Bae]

Add CPU feature conditional FS/GS base access to the relevant helper
functions. That allows accelerating certain FS/GS base operations in
subsequent changes.

Note, that while possible, the user space entry/exit GS base operations are
not going to use the new FSGSBASE instructions. The reason is that it would
require additional storage for the user space value which adds more
complexity to the low level code and experiments have shown marginal
benefit. This may be revisited later but for now the SWAPGS based handling
in the entry code is preserved except for the paranoid entry/exit code.

Suggested-by: Tony Luck <tony.luck@xxxxxxxxx>
Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: Tony Luck <tony.luck@xxxxxxxxx>
Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---

Changes from v8:
* Internalized the interrupt condition check in the helper functions (Andy L.)
* Simplified the GS base read/write helper functions (Tony)
* Massaged the changelog to reflect the helper changes

Changes from v7:
* Added interrupt-related warning messages by Thomas
* Massaged changelog by Thomas
* Used '[FS|GS] base' consistently, instead of '[FS|GS]BASE'
---
 arch/x86/include/asm/fsgsbase.h | 27 +++++++++----------
 arch/x86/kernel/process_64.c    | 58 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 70 insertions(+), 15 deletions(-)

diff --git a/arch/x86/include/asm/fsgsbase.h b/arch/x86/include/asm/fsgsbase.h
index fdd1177..aefd537 100644
--- a/arch/x86/include/asm/fsgsbase.h
+++ b/arch/x86/include/asm/fsgsbase.h
@@ -49,35 +49,32 @@ static __always_inline void wrgsbase(unsigned long gsbase)
 	asm volatile("wrgsbase %0" :: "r" (gsbase) : "memory");
 }
 
+#include <asm/cpufeature.h>
+
 /* Helper functions for reading/writing FS/GS base */
 
 static inline unsigned long x86_fsbase_read_cpu(void)
 {
 	unsigned long fsbase;
 
-	rdmsrl(MSR_FS_BASE, fsbase);
+	if (static_cpu_has(X86_FEATURE_FSGSBASE))
+		fsbase = rdfsbase();
+	else
+		rdmsrl(MSR_FS_BASE, fsbase);
 
 	return fsbase;
 }
 
-static inline unsigned long x86_gsbase_read_cpu_inactive(void)
-{
-	unsigned long gsbase;
-
-	rdmsrl(MSR_KERNEL_GS_BASE, gsbase);
-
-	return gsbase;
-}
-
 static inline void x86_fsbase_write_cpu(unsigned long fsbase)
 {
-	wrmsrl(MSR_FS_BASE, fsbase);
+	if (static_cpu_has(X86_FEATURE_FSGSBASE))
+		wrfsbase(fsbase);
+	else
+		wrmsrl(MSR_FS_BASE, fsbase);
 }
 
-static inline void x86_gsbase_write_cpu_inactive(unsigned long gsbase)
-{
-	wrmsrl(MSR_KERNEL_GS_BASE, gsbase);
-}
+extern unsigned long x86_gsbase_read_cpu_inactive(void);
+extern void x86_gsbase_write_cpu_inactive(unsigned long gsbase);
 
 #endif /* CONFIG_X86_64 */
 
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index af64519..295aa0c 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -329,6 +329,64 @@ static unsigned long x86_fsgsbase_read_task(struct task_struct *task,
 	return base;
 }
 
+unsigned long x86_gsbase_read_cpu_inactive(void)
+{
+	unsigned long gsbase;
+
+	if (static_cpu_has(X86_FEATURE_FSGSBASE)) {
+		bool need_restore = false;
+		unsigned long flags;
+
+		/*
+		 * We read the inactive GS base value by swapping
+		 * to make it the active one. But we cannot allow
+		 * an interrupt while we switch to and from.
+		 */
+		if (!irqs_disabled()) {
+			local_irq_save(flags);
+			need_restore = true;
+		}
+
+		native_swapgs();
+		gsbase = rdgsbase();
+		native_swapgs();
+
+		if (need_restore)
+			local_irq_restore(flags);
+	} else {
+		rdmsrl(MSR_KERNEL_GS_BASE, gsbase);
+	}
+
+	return gsbase;
+}
+
+void x86_gsbase_write_cpu_inactive(unsigned long gsbase)
+{
+	if (static_cpu_has(X86_FEATURE_FSGSBASE)) {
+		bool need_restore = false;
+		unsigned long flags;
+
+		/*
+		 * We write the inactive GS base value by swapping
+		 * to make it the active one. But we cannot allow
+		 * an interrupt while we switch to and from.
+		 */
+		if (!irqs_disabled()) {
+			local_irq_save(flags);
+			need_restore = true;
+		}
+
+		native_swapgs();
+		wrgsbase(gsbase);
+		native_swapgs();
+
+		if (need_restore)
+			local_irq_restore(flags);
+	} else {
+		wrmsrl(MSR_KERNEL_GS_BASE, gsbase);
+	}
+}
+
 unsigned long x86_fsbase_read_task(struct task_struct *task)
 {
 	unsigned long fsbase;
-- 
2.7.4