x86/kexec: Disable FRED earlier in kexec_reloc()

With FRED just as with IDT, it's unsafe to run the exception handlers after
switching stack.

To remove this unsafe window, %cr4 needs clearing earlier.  In turn, we may
need to switch to PCID 0 earlier too in order to be able to clear CR4.PCIDE.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
---
CC: Jan Beulich <JBeulich@suse.com>
CC: Roger Pau Monné <roger.pau@citrix.com>

xen/arch/x86/x86_64/kexec_reloc.S

@@ -54,6 +54,21 @@ FUNC(kexec_reloc, PAGE_SIZE)
         lidt    (%rsp)
         add     $10, %rsp
 
+        /* Move to PCID 0 if necessary, as a prerequisite to clearing CR4.PCIDE */
+        mov     %cr3, %rax
+        test    $0xfff, %eax
+        jz      1f
+        and     $~0xfff, %rax
+        mov     %rax, %cr3
+1:
+
+        /*
+         * Set CR4 to PAE only.  This may disable FRED, which must happen
+         * before switching off Xen's stack.
+         */
+        mov     $X86_CR4_PAE, %eax
+        mov     %rax, %cr4
+
         /*
          * Move to the identity mapped stack.
          *
@@ -86,13 +101,6 @@ FUNC(kexec_reloc, PAGE_SIZE)
         orl     $(X86_CR0_PG | X86_CR0_PE), %eax
         movq    %rax, %cr0
 
-        /*
-         * Set cr4 to a known state:
-         *  - physical address extension enabled
-         */
-        movl    $X86_CR4_PAE, %eax
-        movq    %rax, %cr4
-
         movq    %rdx, %rdi
         call    relocate_pages