[fedora-virt] f10 x86_64 xen VM guests fail to boot on f8 host (guest setting NX bit in L1 PTE?)

[Mark McLoughlin]

(Jeremy/Ian - here's some more info on the bug reported here:

  http://lists.xensource.com/archives/html/xen-devel/2009-01/msg00176.html
)

Hi Jon/Phill,

Thanks for all the info.

Here's the important bits:

  1) Host kernel is 2.6.21.7-5.fc8xen, that means the hypervisor is 
     xen-3.1.4

  2) The guest kernel is 2.6.27.5-117.fc10.x86_64

  3) Phill points out the faulting instruction is UD2. That just means 
     the guest kernel is hitting a BUG() assertion. See /asm-x86/bug.h:

       #define BUG()                                                   \
       do {                                                            \
               asm volatile("ud2");                                    \
               for (;;) ;                                              \
       } while (0)

   4) The backtrace shows the fault happens in set_page_prot()

   5) Jon's dmesg contains:

        (XEN) mm.c:1362:d46 Bad L1 flags 800000

That means the guest is faulting here:

static void set_page_prot(void *addr, pgprot_t prot)
{
....
        if (HYPERVISOR_update_va_mapping((unsigned long)addr, pte, 0))
                BUG();
}

because the PTE update is failing in the HV here:

static int mod_l1_entry(l1_pgentry_t *pl1e, l1_pgentry_t nl1e, 
                        unsigned long gl1mfn)
{
...
        if ( unlikely(l1e_get_flags(nl1e) & L1_DISALLOW_MASK) )
        {
            MEM_LOG("Bad L1 flags %x",
                    l1e_get_flags(nl1e) & L1_DISALLOW_MASK);
            return 0;
        }
...
}

the PTE flags are 800000 which corresponds to:

#define _PAGE_NX_BIT (1U<<23)

Jon/Phill - can one of you two file a bug (bugzilla.redhat.com) with all
this info?

Thanks,
Mark.

Previous posts, for reference:

http://www.redhat.com/archives/fedora-xen/2009-January/thread.html#00022
http://www.redhat.com/archives/fedora-virt/2009-January/thread.html#00013