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@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit 3ff1030ea7)

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);

xen/arch/x86/traps.c

@@ -124,7 +124,7 @@ 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();