[xen staging] x86/fsgsbase: Make SHADOW_GS accesses safe under FRED

[patchbot@xxxxxxx]

commit 3ff1030ea7d40e62edfe09b44dbdba44def5072a
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Mon Aug 18 15:44:44 2025 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Sat Feb 14 01:01:33 2026 +0000

    x86/fsgsbase: Make SHADOW_GS accesses safe under FRED
    
    Under FRED, the SWAPGS instruction is disallowed.  Therefore we must use the
    MSR path instead.
    
    read_registers() is in the show_registers() path, so this change allows Xen 
to
    render it's current state without suffering #UD (and recursing until the 
stack
    guard page is hit).
    
    All hardware with FRED is expected to have some kind of non-serialising 
access
    to these registers.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/include/asm/fsgsbase.h | 8 ++++++--
 xen/arch/x86/traps.c                | 2 +-
 2 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/include/asm/fsgsbase.h 
b/xen/arch/x86/include/asm/fsgsbase.h
index 24862a6bfe..5faa3a3243 100644
--- a/xen/arch/x86/include/asm/fsgsbase.h
+++ b/xen/arch/x86/include/asm/fsgsbase.h
@@ -79,7 +79,9 @@ static inline unsigned long read_gs_base(void)
 
 static inline unsigned long read_gs_shadow(void)
 {
-    if ( read_cr4() & X86_CR4_FSGSBASE )
+    unsigned long cr4 = read_cr4();
+
+    if ( !(cr4 & X86_CR4_FRED) && (cr4 & X86_CR4_FSGSBASE) )
         return __rdgs_shadow();
     else
         return rdmsr(MSR_SHADOW_GS_BASE);
@@ -103,7 +105,9 @@ static inline void write_gs_base(unsigned long base)
 
 static inline void write_gs_shadow(unsigned long base)
 {
-    if ( read_cr4() & X86_CR4_FSGSBASE )
+    unsigned long cr4 = read_cr4();
+
+    if ( !(cr4 & X86_CR4_FRED) && (cr4 & X86_CR4_FSGSBASE) )
         __wrgs_shadow(base);
     else
         wrmsrns(MSR_SHADOW_GS_BASE, base);
diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
index f621b99a5f..3c01c5eb53 100644
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -123,7 +123,7 @@ static void read_registers(struct extra_state *state)
      * invalidate the read_cr4() address calculation (really cpu_info->cr4
      * under the hood), forcing the cr4 check to be re-evaluated every time.
      */
-    if ( state->cr4 & X86_CR4_FSGSBASE )
+    if ( !(state->cr4 & X86_CR4_FRED) && (state->cr4 & X86_CR4_FSGSBASE) )
     {
         state->fsb = __rdfsbase();
         state->gsb = __rdgsbase();
--
generated by git-patchbot for /home/xen/git/xen.git#staging