[PATCH v9 14/36] x86/fred: Disallow the swapgs instruction when FRED is enabled

From: "H. Peter Anvin (Intel)" <hpa@zytor.com>

The FRED architecture establishes the full supervisor/user through:
1) FRED event delivery from ring 3 swaps the value of the GS base
   address and that of the IA32_KERNEL_GS_BASE MSR.
2) ERETU swaps the value of the GS base address and that of the
   IA32_KERNEL_GS_BASE MSR.
3) LKGS is already upstreamed and automatically enabled with FRED to
   load the GS base address directly into the IA32_KERNEL_GS_BASE MSR
   instead of the GS segment’s descriptor cache.

As a result, there is no need to SWAPGS away from the kernel GS base,
i.e., the swapgs instruction is no longer needed when FRED is enabled,
thus is disallowed. Otherwise it causes #UD.

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Tested-by: Shan Kang <redacted>
Signed-off-by: Xin Li <redacted>
---

Changes since v8:
* Explain why writing directly to the IA32_KERNEL_GS_BASE MSR is
  doing the right thing (Thomas Gleixner).
---
 arch/x86/kernel/process_64.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 265ab8fcb146..6d5fed29f552 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c

@@ -166,7 +166,8 @@ static noinstr unsigned long __rdgsbase_inactive(void)
 
 	lockdep_assert_irqs_disabled();
 
-	if (!cpu_feature_enabled(X86_FEATURE_XENPV)) {
+	if (!cpu_feature_enabled(X86_FEATURE_FRED) &&
+	    !cpu_feature_enabled(X86_FEATURE_XENPV)) {
 		native_swapgs();
 		gsbase = rdgsbase();
 		native_swapgs();

@@ -191,7 +192,8 @@ static noinstr void __wrgsbase_inactive(unsigned long gsbase)
 {
 	lockdep_assert_irqs_disabled();
 
-	if (!cpu_feature_enabled(X86_FEATURE_XENPV)) {
+	if (!cpu_feature_enabled(X86_FEATURE_FRED) &&
+	    !cpu_feature_enabled(X86_FEATURE_XENPV)) {
 		native_swapgs();
 		wrgsbase(gsbase);
 		native_swapgs();

-- 
2.34.1