[tip: x86/fsgsbase] x86/fsgsbase: Fix Xen PV support

[tip-bot2 for Andy Lutomirski]

The following commit has been merged into the x86/fsgsbase branch of tip:

Commit-ID:     d029bff60aa6c7eab281d52602b6a7a971615324
Gitweb:        https://git.kernel.org/tip/d029bff60aa6c7eab281d52602b6a7a971615324
Author:        Andy Lutomirski <luto@xxxxxxxxxx>
AuthorDate:    Fri, 26 Jun 2020 10:24:30 -07:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Wed, 01 Jul 2020 15:27:20 +02:00

x86/fsgsbase: Fix Xen PV support

On Xen PV, SWAPGS doesn't work.  Teach __rdfsbase_inactive() and
__wrgsbase_inactive() to use rdmsrl()/wrmsrl() on Xen PV.  The Xen
pvop code will understand this and issue the correct hypercalls.

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/f07c08f178fe9711915862b656722a207cd52c28.1593192140.git.luto@xxxxxxxxxx

---
 arch/x86/kernel/process_64.c | 24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index cb8e37d..e14476f 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -163,9 +163,15 @@ static noinstr unsigned long __rdgsbase_inactive(void)
 
 	lockdep_assert_irqs_disabled();
 
-	native_swapgs();
-	gsbase = rdgsbase();
-	native_swapgs();
+	if (!static_cpu_has(X86_FEATURE_XENPV)) {
+		native_swapgs();
+		gsbase = rdgsbase();
+		native_swapgs();
+	} else {
+		instrumentation_begin();
+		rdmsrl(MSR_KERNEL_GS_BASE, gsbase);
+		instrumentation_end();
+	}
 
 	return gsbase;
 }
@@ -182,9 +188,15 @@ static noinstr void __wrgsbase_inactive(unsigned long gsbase)
 {
 	lockdep_assert_irqs_disabled();
 
-	native_swapgs();
-	wrgsbase(gsbase);
-	native_swapgs();
+	if (!static_cpu_has(X86_FEATURE_XENPV)) {
+		native_swapgs();
+		wrgsbase(gsbase);
+		native_swapgs();
+	} else {
+		instrumentation_begin();
+		wrmsrl(MSR_KERNEL_GS_BASE, gsbase);
+		instrumentation_end();
+	}
 }
 
 /*