[PATCH 4/4] x86/kexec: Disable FRED earlier in kexec_reloc()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>

The pagetable switch cannot easily be moved to be earlier, as that leaves a
period of time where FRED is enabled but all stack pointers point to
non-existent mappings.
---
 xen/arch/x86/x86_64/kexec_reloc.S | 22 +++++++++++++++-------
 1 file changed, 15 insertions(+), 7 deletions(-)

diff --git a/xen/arch/x86/x86_64/kexec_reloc.S 
b/xen/arch/x86/x86_64/kexec_reloc.S
index 7a6dd2cbe736..81da81a827de 100644
--- a/xen/arch/x86/x86_64/kexec_reloc.S
+++ b/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
 
-- 
2.39.5