x86/svm: Always sync guest CR2 on VMExit

Under SVM, there are two copies of guest CR2.  One is v->arch.hvm.guest_cr[2]
and one is in the VMCB.

Xen doesn't intercept CR2 accesses, so this mostly goes unnoticed; hardware
loads and saves the guest CR2 in the VMCB across VMRUN/VMExit.

For HAP guests (where #PF is not intercepted, and therefore we don't typically
inject #PF either), this causes the guest CR2 value to be lost on migrate.  As
migration is cooperative and not done from the #PF handler, this also goes
unnoticed by guests.

It also means that an emulated MOV-from-CR2 reads a stale value.

Reported-by: Stefano Stabellini <sstabellini@kernel.org>
Fixes: d1bd157fbc ("Big merge the HVM full-virtualisation abstractions.")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Tested-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Teddy Astie <teddy.astie@vates.tech>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/hvm/svm/svm.c

@@ -2505,6 +2505,7 @@ void asmlinkage svm_vmexit_handler(void)
     hvm_sanitize_regs_fields(
         regs, !(vmcb_get_efer(vmcb) & EFER_LMA) || !(vmcb->cs.l));
 
+    v->arch.hvm.guest_cr[2] = vmcb_get_cr2(vmcb);
     if ( paging_mode_hap(v->domain) )
         v->arch.hvm.guest_cr[3] = v->arch.hvm.hw_cr[3] = vmcb_get_cr3(vmcb);