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



Hi,

I can confirm this. With NX enabled in BIOS the domU boots fine (tested with 2.6.28).

If I disable NX in BIOS the domU will crash.

- Valtteri Kiviniemi

Virtualization kirjoitti:
Hi Ian,

Indeed nx is on one and not the other! However, that doesn't help...

Broken CPU:
/proc/cpuinfo:
model name      :                   Intel(R) Xeon(TM) CPU 3.00GHz
flags           : fpu tsc msr pae mce cx8 apic mtrr mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm syscall lm constant_tsc up pni monitor ds_cpl cid cx16 xtpr

Good CPU:
model name      : AMD Athlon(tm) 64 X2 Dual Core Processor 3600+
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt rdtscp lm 3dnowext 3dnow rep_good pni cx16 lahf_lm cmp_legacy svm extapic cr8_legacy 3dnowprefetch

So, ran it again with noexec=0,

  [ Minimal BASH-like line editing is supported.
    ESC at any time cancels.  ENTER at any time accepts your changes. ]


  > kernel /vmlinuz-2.6.27.9-159.fc10.x86_64 ro root=LABEL=/ selinux=0 noipv6 nomodeset noexec=off

Results:

[ root office64 xen ]# /usr/lib64/xen/bin/xenctx -s System.map-2.6.27.9-159.fc10.x86_64 119
rip: ffffffff8100b8a2 set_page_prot+0x6d
rsp: ffffffff81575f08
rax: ffffffea   rbx: 000016e4   rcx: 00000055   rdx: 00000000
rsi: 800000014a293061   rdi: ffffffff816e4000   rbp: ffffffff81575f68
 r8: 0000000f    r9: ffffffff817ee350   r10: ffffffff817ee550   r11: 00000010
r12: ffffffff816e4000   r13: 800000014a293061   r14: 8000000000000161   r15: 00002c00
 cs: 0000e033    ds: 00000000    fs: 00000000    gs: 00000000

Stack:
 0000000000000055 0000000000000010 ffffffff8100b8a2 000000010000e030
 0000000000010082 ffffffff81575f48 000000000000e02b ffffffff8100b89e
 0000000000000200 ffffffff816e7000 0000000000000800 0000000000000016
 ffffffff81575ff8 ffffffff815a5c60 0000000000002c00 0000000000000000

Code:
df 54 1d 00 4c 89 e7 4c 89 ee 31 d2 e8 22 d9 ff ff 85 c0 74 04 <0f> 0b eb fe 5b 41 5c 41 5d 41 5e

Call Trace:
  [<ffffffff8100b8a2>] set_page_prot+0x6d <--
  [<ffffffff8100b8a2>] set_page_prot+0x6d
  [<ffffffff8100b89e>] set_page_prot+0x69
  [<ffffffff815a5c60>] xen_start_kernel+0x5dd

Battling with bugzilla trying to get a new account. It doesn't like me :-(

Might have to leave it up to Jon to do the bugzilla thing.

Cheers
Phill.

On Tue, 2009-01-20 at 17:03 +0000, Ian Campbell wrote:
(resending with original xen-devel thread participants on CC, please
reply to this subthread, I'll forward you guys Mark's original mail in a
second)

On Tue, 2009-01-20 at 10:27 +0000, Mark McLoughlin wrote:
       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)
At least in xen-unstable (and I think for much longer) L1_DISALLOW_MASK
contains _PAGE_NX_BIT dynamically depending on the processor
capabilities.

        #define _PAGE_NX     (cpu_has_nx ? _PAGE_NX_BIT : 0)
        ...
        /*
         * Disallow unused flag bits plus PAT/PSE, PCD, PWT and GLOBAL.
         * Permit the NX bit if the hardware supports it.
         */
        #define BASE_DISALLOW_MASK (0xFFFFF198U & ~_PAGE_NX)
#define L1_DISALLOW_MASK (BASE_DISALLOW_MASK | _PAGE_GNTTAB)

Does the hardware support NX? What does /proc/cpuinfo in dom0 think?

The guest kernel should be setting up __supported_pte_mask appropriately
to match the hardware and hence shouldn't be using NX if it isn't
available. There's a command line option to force NX, can you try
noexec=off on the guest command line.

My guess would be that the guest is getting a wrong EFER from
somewhere...

Ian.





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