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Subject[PATCH v9 08/17] x86/entry/64: Document GSBASE handling in the paranoid path
On FSGSBASE systems, the way to handle GS base in the paranoid path is
different from the existing SWAPGS-based entry/exit path handling. Document
the reason and what has to be done for FSGSBASE enabled systems.

Signed-off-by: Chang S. Bae <>
Reviewed-by: Tony Luck <>
Cc: Thomas Gleixner <>
Cc: Borislav Petkov <>
Cc: Andy Lutomirski <>
Cc: H. Peter Anvin <>
Cc: Dave Hansen <>
Cc: Tony Luck <>
Cc: Andi Kleen <>

Changes from v8: none

Changes from v7:
* Massaged doc and changelog by Thomas
* Used 'GS base' consistently, instead of 'GSBASE'
Documentation/x86/entry_64.rst | 9 +++++++++
1 file changed, 9 insertions(+)

diff --git a/Documentation/x86/entry_64.rst b/Documentation/x86/entry_64.rst
index a48b3f6..0499a40 100644
--- a/Documentation/x86/entry_64.rst
+++ b/Documentation/x86/entry_64.rst
@@ -108,3 +108,12 @@ We try to only use IST entries and the paranoid entry code for vectors
that absolutely need the more expensive check for the GS base - and we
generate all 'normal' entry points with the regular (faster) paranoid=0
+On FSGSBASE systems, however, user space can set GS without kernel
+interaction. It means the value of GS base itself does not imply anything,
+whether a kernel value or a user space value. So, there is no longer a safe
+way to check whether the exception is entering from user mode or kernel
+mode in the paranoid entry code path. So the GS base value needs to be read
+out, saved and the kernel GS base value written. On exit, the saved GS base
+value needs to be restored unconditionally. The non-paranoid entry/exit
+code still uses SWAPGS unconditionally as the state is known.
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