Description of problem:
kernel-xen boot up hang there after enabled iommu on a VT-d enabled machine. Bellow is the serial console of the boot process:

 __  __            _____  _   ____     _  __   _  _        _ ____
 \ \/ /___ _ __   |___ / / | |___ \   / |/ /_ | || |   ___| | ___|
  \  // _ \ '_ \    |_ \ | |   __) |__| | '_ \| || |_ / _ \ |___ \
  /  \  __/ | | |  ___) || |_ / __/|__| | (_) |__   _|  __/ |___) |
 /_/\_\___|_| |_| |____(_)_(_)_____|  |_|\___/   |_|(_)___|_|____/

 http://www.cl.cam.ac.uk/netos/xen
 University of Cambridge Computer Laboratory

 Xen version 3.1.2-164.el5 (mockbuild) (gcc version 4.1.2 20080704 (Red Hat 4.1.2-46)) Tue Aug 18 15:50:30 EDT 2009
 Latest ChangeSet: unavailable

(XEN) Command line: iommu=1 com1=115200n8 noreboot
(XEN) Video information:
(XEN)  VGA is text mode 80x25, font 8x16
(XEN)  VBE/DDC methods: V2; EDID transfer time: 1 seconds
(XEN) Disc information:
(XEN)  Found 1 MBR signatures
(XEN)  Found 1 EDD information structures
(XEN) Xen-e820 RAM map:
(XEN)  0000000000000000 - 000000000009fc00 (usable)
(XEN)  00000000000f0000 - 0000000000100000 (reserved)
(XEN)  0000000000100000 - 00000000cfdffc00 (usable)
(XEN)  00000000cfdffc00 - 00000000cfe53c00 (ACPI NVS)
(XEN)  00000000cfe53c00 - 00000000cfe55c00 (ACPI data)
(XEN)  00000000cfe55c00 - 00000000d0000000 (reserved)
(XEN)  00000000e0000000 - 00000000f0000000 (reserved)
(XEN)  00000000fec00000 - 00000000fed00400 (reserved)
(XEN)  00000000fed20000 - 00000000feda0000 (reserved)
(XEN)  00000000fee00000 - 00000000fef00000 (reserved)
(XEN)  00000000ffb00000 - 0000000100000000 (reserved)
(XEN)  0000000100000000 - 000000012c000000 (usable)
(XEN) System RAM: 4029MB (4126328kB)
(XEN)   >>> ERROR: Invalid checksum
(XEN) Xen heap: 13MB (13856kB)
(XEN) Domain heap initialised: DMA width 32 bits
(XEN) Processor #0 7:7 APIC version 20
(XEN) Processor #1 7:7 APIC version 20
(XEN) Processor #2 7:7 APIC version 20
(XEN) Processor #3 7:7 APIC version 20
(XEN) IOAPIC[0]: apic_id 8, version 32, address 0xfec00000, GSI 0-23
(XEN) Enabling APIC mode:  Flat.  Using 1 I/O APICs
(XEN) Intel VT-d has been enabled
(XEN) Using scheduler: SMP Credit Scheduler (credit)
(XEN) Detected 2660.082 MHz processor.
(XEN) HVM: VMX enabled
(XEN) VMX: MSR intercept bitmap enabled
(XEN) CPU0: Intel(R) Core(TM)2 Quad CPU    Q9400  @ 2.66GHz stepping 0a
(XEN) Booting processor 1/1 eip 90000
(XEN) CPU1: Intel(R) Core(TM)2 Quad CPU    Q9400  @ 2.66GHz stepping 0a
(XEN) Booting processor 2/2 eip 90000
(XEN) CPU2: Intel(R) Core(TM)2 Quad CPU    Q9400  @ 2.66GHz stepping 0a
(XEN) Booting processor 3/3 eip 90000
(XEN) CPU3: Intel(R) Core(TM)2 Quad CPU    Q9400  @ 2.66GHz stepping 0a
(XEN) Total of 4 processors activated.
(XEN) ENABLING IO-APIC IRQs
(XEN)  -> Using new ACK method
(XEN) Platform timer overflows in 14998 jiffies.
(XEN) Platform timer is 14.318MHz HPET
(XEN) Brought up 4 CPUs
(XEN) ----[ Xen-3.1.2-164.el5  x86_64  debug=n  Not tainted ]----
(XEN) CPU:    0
(XEN) RIP:    e008:[<ffff828c80124110>] clear_fault_bits+0x20/0xa0
(XEN) RFLAGS: 0000000000010206   CONTEXT: hypervisor
(XEN) rax: ffff828bfff5bff0   rbx: 0000000000000090   rcx: ffff828bfff58000
(XEN) rdx: 00000000ffffffff   rsi: 00000000ffffffff   rdi: ffff8300cfbfee00
(XEN) rbp: ffff8300cfbfee00   rsp: ffff828c802f7e10   r8:  ffff8300cfbfee00
(XEN) r9:  ffff828c801e94e0   r10: 00000000000049d7   r11: 0000000000000000
(XEN) r12: ffff8300cef79880   r13: 0000000000000206   r14: ffff828c801c37c0
(XEN) r15: 0000000001000000   cr0: 000000008005003b   cr4: 00000000000026b0
(XEN) cr3: 00000000ceefb000   cr2: ffff828bfff5bff8
(XEN) ds: 0000   es: 0000   fs: 0000   gs: 0000   ss: 0000   cs: e008
(XEN) Xen stack trace from rsp=ffff828c802f7e10:
(XEN)    ffff828c801246a4 000000000000001f ffff8300cecfa080 ffff828c801c3790
(XEN)    ffff828c801c37c0 0000000000100000 ffff828c80124f55 0000000000000040
(XEN)    ffff828c8012c86b 000000000000001f 00000000ffffffed ffff828c801e8068
(XEN)    0000000000000003 0000000000000100 ffff828c80122947 ffff828c801e7fc0
(XEN)    ffff828c801dcf04 ffff8300ceefbff8 00000000001cd476 00000000001cd4db
(XEN)    0000000000000000 ffff83000002ccc0 ffff830000021a40 0000000800000000
(XEN)    000000010000006e 0000000000000003 00000000000002f8 0000000000000000
(XEN)    0000000000000000 0000000000067e4c 0000000000000000 0000000000000000
(XEN)    0000000000000000 0000000000000000 ffff828c801000b5 0000000000000000
(XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
(XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
(XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
(XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
(XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
(XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
(XEN)    0000000000000000 ffff8300cec02080
(XEN) Xen call trace:
(XEN)    [<ffff828c80124110>] clear_fault_bits+0x20/0xa0
(XEN)    [<ffff828c801246a4>] init_vtd_hw+0x174/0x250
(XEN)    [<ffff828c80124f55>] intel_vtd_setup+0x2f5/0x420
(XEN)    [<ffff828c8012c86b>] __find_next_bit+0x4b/0x60
(XEN)    [<ffff828c80122947>] iommu_setup+0x27/0xb0
(XEN)    [<ffff828c801dcf04>] __start_xen+0x2164/0x2410
(XEN)    [<ffff828c801000b5>] __high_start+0xa1/0xa3
(XEN)
(XEN) Pagetable walk from ffff828bfff5bff8:
(XEN)  L4[0x105] = 00000000cef01027 5555555555555555
(XEN)  L3[0x02f] = 0000000000104063 5555555555555555
(XEN)  L2[0x1ff] = 0000000000105063 5555555555555555
(XEN)  L1[0x15b] = 0000000000000000 ffffffffffffffff
(XEN)
(XEN) ****************************************
(XEN) Panic on CPU 0:
(XEN) FATAL PAGE FAULT
(XEN) [error_code=0000]
(XEN) Faulting linear address: ffff828bfff5bff8
(XEN) ****************************************
(XEN)
(XEN) Manual reset required ('noreboot' specified)



Version-Release number of selected component (if applicable):
xen-3.0.3-94.el5
kernel-xen-2.6.18-164.el5


How reproducible:
Always

Steps to Reproduce:
1. Enable VT-d in BIOS
2. Enable IOMMU in grub.conf: 
   kernel /xen.gz-2.6.18-164.el5 iommu=1

  
Actual results:
xen-kernel will hang and reboot 


Expected results:
xen-kernel should boot successfully 

Additional info:
Bug 541209 - kernel-xen boot up hang there after enabled iommu on AMD Magny Cours system