From: "H. Peter Anvin (Intel)" <h...@zytor.com>
SWAPGS is no longer needed thus NOT allowed with FRED because FRED
transitions ensure that an operating system can _always_ operate
with its own GS base address:
- For events that occur in ring 3, FRED event delivery swaps the GS
base address with the IA32_KERNEL_GS_BASE MSR.
- ERETU (the FRED transition that returns to ring 3) also swaps the
GS base address with the IA32_KERNEL_GS_BASE MSR.
And the operating system can still setup the GS segment for a user
thread without the need of loading a user thread GS with:
- Using LKGS, available with FRED, to modify other attributes of the
GS segment without compromising its ability always to operate with
its own GS base address.
- Accessing the GS segment base address for a user thread as before
using RDMSR or WRMSR on the IA32_KERNEL_GS_BASE MSR.
Note, LKGS loads the GS base address into the IA32_KERNEL_GS_BASE MSR
instead of the GS segment's descriptor cache. As such, the operating
system never changes its runtime GS base address.
Signed-off-by: H. Peter Anvin (Intel) <h...@zytor.com>
Tested-by: Shan Kang <shan.k...@intel.com>
Signed-off-by: Xin Li <xin3...@intel.com>
---
Change since v12:
* Use the ASCII char ' (char number 0x27), instead of its unicode char
(Borislav Petkov).
Change 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 | 27 +++++++++++++++++++++++++--
1 file changed, 25 insertions(+), 2 deletions(-)
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index b924477c5ba8..7f66c0b14de6 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -166,7 +166,29 @@ static noinstr unsigned long __rdgsbase_inactive(void)
lockdep_assert_irqs_disabled();
- if (!cpu_feature_enabled(X86_FEATURE_XENPV)) {
+ /*
+ * SWAPGS is no longer needed thus NOT allowed with FRED because
+ * FRED transitions ensure that an operating system can _always_
+ * operate with its own GS base address:
+ * - For events that occur in ring 3, FRED event delivery swaps
+ * the GS base address with the IA32_KERNEL_GS_BASE MSR.
+ * - ERETU (the FRED transition that returns to ring 3) also swaps
+ * the GS base address with the IA32_KERNEL_GS_BASE MSR.
+ *
+ * And the operating system can still setup the GS segment for a
+ * user thread without the need of loading a user thread GS with:
+ * - Using LKGS, available with FRED, to modify other attributes
+ * of the GS segment without compromising its ability always to
+ * operate with its own GS base address.
+ * - Accessing the GS segment base address for a user thread as
+ * before using RDMSR or WRMSR on the IA32_KERNEL_GS_BASE MSR.
+ *
+ * Note, LKGS loads the GS base address into the IA32_KERNEL_GS_BASE
+ * MSR instead of the GS segment’s descriptor cache. As such, the
+ * operating system never changes its runtime GS base address.
+ */
+ if (!cpu_feature_enabled(X86_FEATURE_FRED) &&
+ !cpu_feature_enabled(X86_FEATURE_XENPV)) {
native_swapgs();
gsbase = rdgsbase();
native_swapgs();
@@ -191,7 +213,8 @@ 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();
--
2.43.0
