xpti: fix bug in double fault handling

When entering the hypervisor via the double fault handler resetting
xen_cr3 was missing. This led to switching to pv_cr3 when returning
from the next following exception, so repair this in order to allow
exception handling to work even after a double fault.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Tested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/x86_64/entry.S

@@ -767,12 +767,14 @@ ENTRY(double_fault)
         /* WARNING! `ret`, `call *`, `jmp *` not safe before this point. */
 
         mov   STACK_CPUINFO_FIELD(xen_cr3)(%r14), %rbx
-        test  %rbx, %rbx
+        neg   %rbx
         jz    .Ldblf_cr3_okay
         jns   .Ldblf_cr3_load
+        mov   %rbx, STACK_CPUINFO_FIELD(xen_cr3)(%r14)
         neg   %rbx
 .Ldblf_cr3_load:
         mov   %rbx, %cr3
+        movq $0, STACK_CPUINFO_FIELD(xen_cr3)(%r14)
 .Ldblf_cr3_okay:
 
         movq  %rsp,%rdi