From: "H. Peter Anvin (Intel)" <hpa@xxxxxxxxx>
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@xxxxxxxxx>
Tested-by: Shan Kang <shan.kang@xxxxxxxxx>
Signed-off-by: Xin Li <xin3.li@xxxxxxxxx>
---
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
