Subject: x86/hvm: Disallow the creation of HVM domains without Local APIC emulation
From: Andrew Cooper <[email-masked]>

There are multiple problems, not necesserily limited to:

 * Guests which configure event channels via hvmop_set_evtchn_upcall_vector(),
   or which hit %cr8 emulation will cause Xen to fall over a NULL vlapic->regs
   pointer.

 * On Intel hardware, disabling the TPR_SHADOW execution control without
   reenabling CR8_{LOAD,STORE} interception means that the guests %cr8
   accesses interact with the real TPR.  Amongst other things, setting the
   real TPR to 0xf blocks even IPIs from interrupting this CPU.

 * On hardware which sets up the use of Interrupt Posting, including
   IOMMU-Posting, guests run without the appropriate non-root configuration,
   which at a minimum will result in dropped interrupts.

Whether no-LAPIC mode is of any use at all remains to be seen.

This is XSA-256.

Reported-by: Ian Jackson <[email-masked]>
Reviewed-by: Roger Pau Monné <[email-masked]>
Signed-off-by: Andrew Cooper <[email-masked]>
Reviewed-by: Jan Beulich <[email-masked]>

diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index 8817263..91b9ab7 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -557,11 +557,9 @@ int arch_domain_create(struct domain *d, unsigned int domcr_flags,
             return -EINVAL;
         }
 
-        /* PVHv2 guests can request emulated APIC. */
-        if ( emflags &&
-            (is_hvm_domain(d) ? ((emflags != XEN_X86_EMU_ALL) &&
-                                 (emflags != XEN_X86_EMU_LAPIC)) :
-                                (emflags != XEN_X86_EMU_PIT)) )
+        if ( is_hvm_domain(d) ? ((emflags != XEN_X86_EMU_ALL) &&
+                                 (emflags != XEN_X86_EMU_LAPIC))
+                              : (emflags && emflags != XEN_X86_EMU_PIT) )
         {
             printk(XENLOG_G_ERR "d%d: Xen does not allow %s domain creation "
                    "with the current selection of emulators: %#x\n",