Thanks for the info Dave - maybe this belongs in the appropriate Xen list, but this code seems fundamentally broken to me for a couple of reasons: 1. I'm pretty sure that the address passed into read_mem is a physical address NOT a machine-address and yet the code in is_local_lowmem seems to treat this as an mfn (at least, I would assume that mfn_to_local_pfn would be doing so) 2. That aside, it just seems wrong to me to break the operation of /dev/mem so that the one thing it _cant_ return is low memory for the guest! Time for a Xen bug I think! Simon ________________________________ From: crash-utility-bounces(a)redhat.com [mailto:crash-utility-bounces@redhat.com] On Behalf Of Dave Anderson Sent: Monday, May 08, 2006 10:36 AM To: Discussion list for crash utility usage, maintenance and development Subject: Re: [Crash-utility] Problems debugging live system with 4.0-2.24 crash "Graham, Simon" wrote: I've been experimenting with the latest crash version on a system running Xen (3.0.2-2 with kernel 2.6.16) and am experiencing problems when attempting to debug the system live; debugging of saved guest files and guest crash dumps works well, it's just live debugging of dom0 that is problematical. The symptom is that I get the error: WARNING: cannot read linux_banner string When I debug crash, I can see that it is finding the correct unstripped kernel from which it finds the address of the linux_banner variable _but_ when it attempts to read the string from /dev/mem it gets all zeroes back. I have confirmed that the address is correct by using gdb against /proc/kcore - gdb reads the string quite nicely from the virtual address (happens to be 0xc0450000, I can see that crash translates this to physical address 450000 and gets all zeroes when it reads this location from /dev/mem). Hi Graham, The xen /dev/mem driver read_mem() function does this: if ((v = ioremap(p, sz)) == NULL) { /* * Some programs (e.g., dmidecode) groove off into weird RAM * areas where no tables can possibly exist (because Xen will * have stomped on them!). These programs get rather upset if * we let them know that Xen failed their access, so we fake * out a read of all zeroes. :-) */ if (clear_user(buf, count)) return -EFAULT; read += count; break; } when ioremap() returns NULL. Here's the NULL-returner in __ioremap(): /* * Don't allow anybody to remap normal RAM that we're using.. */ if (is_local_lowmem(phys_addr)) { char *t_addr, *t_end; struct page *page; t_addr = bus_to_virt(phys_addr); t_end = t_addr + (size - 1); for(page = virt_to_page(t_addr); page <= virt_to_page(t_end); page++) if(!PageReserved(page)) return NULL; domid = DOMID_SELF; } So /dev/mem cannot be used. So to get crash to work on a live x86 xen0 system (FC5), I've had to use the RHEL4 crash.c module (/dev/crash) to make an end-run around /dev/mem. RHEL4's x86 and x86_64 /dev/mem driver only allows reads of the first 256 pages of physical memory. And now Xen has the restriction above. FC5 has a CONFIG_CRASH configurable: $ diff kernel-2.6.15-i686-guest.config.orig kernel-2.6.15-i686-guest.config 2526c2526 < # CONFIG_CRASH is not set --- 2965d2964 < # CONFIG_CRASH is not set $ If you're building from an upstream source, you could build the crash.c module from FC5/RHEL4 into that kernel, or perhaps patch the xen /dev/mem driver above to remove the local_lowmem() restriction, or something like that... Dave I'm pretty sure that the various files in /boot for dom0 all match the running system. Output from `crash -d2' attached in case its useful... So - the question is - what's the obvious thing I am missing! Thanks, Simon crash 4.0-2.24 Copyright (C) 2002, 2003, 2004, 2005, 2006 Red Hat, Inc. Copyright (C) 2004, 2005, 2006 IBM Corporation Copyright (C) 1999-2006 Hewlett-Packard Co Copyright (C) 2005 Fujitsu Limited Copyright (C) 2005 NEC Corporation Copyright (C) 1999, 2002 Silicon Graphics, Inc. Copyright (C) 1999, 2000, 2001, 2002 Mission Critical Linux, Inc. This program is free software, covered by the GNU General Public License, and you are welcome to change it and/or distribute copies of it under certain conditions. Enter "help copying" to see the conditions. This program has absolutely no warranty. Enter "help warranty" for details. find_booted_kernel: search for [Linux version 2.6.16-xen0 (sgraham(a)anna.sn.stratus.com) (gcc version 3.4.4 20050721 (Red Hat 3.4.4-2)) #3 Sat May 6 19:04:09 EDT 2006 ] mount_points[0]: / (83a1ac0) mount_points[1]: /proc (83a1ad0) mount_points[2]: /dev (83a1ae0) mount_points[3]: / (83a1af0) mount_points[4]: /dev (83a1b00) mount_points[5]: /proc (83a1b10) mount_points[6]: /sys (83a1b20) mount_points[7]: /dev/pts (83a1b30) mount_points[8]: /boot (83a1b48) mount_points[9]: /dev/shm (83a1b58) searchdirs[9]: /usr/lib/debug/lib/modules/2.6.16-xen0/ searchdirs[0]: /usr/src/linux/ searchdirs[1]: /boot/ searchdirs[2]: /boot/efi/redhat searchdirs[3]: /boot/efi/EFI/redhat searchdirs[4]: / searchdirs[5]: /usr/src/kernels/ searchdirs[6]: /usr/src/debug/ searchdirs[7]: /usr/src/redhat/BUILD/kernel-2.6.16/linux/ searchdirs[8]: /usr/src/redhat/BUILD/kernel-2.6.16/linux-2.6.16/ find_booted_kernel: check: /boot/xen-syms-3.0.2-2 find_booted_kernel: check: /boot/vmlinux-syms-2.6.16-xenU find_booted_kernel: check: /boot/vmlinux-syms-2.6.16-xen0 find_booted_kernel: found: /boot/vmlinux-syms-2.6.16-xen0 get_live_memory_source: /dev/mem /proc/version: Linux version 2.6.16-xen0 (sgraham(a)anna.sn.stratus.com) (gcc version 3.4.4 20050721 (Red Hat 3.4.4-2)) #3 Sat May 6 19:04:09 EDT 2006 /boot/vmlinux-syms-2.6.16-xen0: Linux version 2.6.16-xen0 (sgraham(a)anna.sn.stratus.com) (gcc version 3.4.4 20050721 (Red Hat 3.4.4-2)) #3 Sat May 6 19:04:09 EDT 2006 gdb /boot/vmlinux-syms-2.6.16-xen0 GNU gdb 6.1 Copyright 2004 Free Software Foundation, Inc. GDB is free software, covered by the GNU General Public License, and you are welcome to change it and/or distribute copies of it under certain conditions. Type "show copying" to see the conditions. There is absolutely no warranty for GDB. Type "show warranty" for details. This GDB was configured as "i686-pc-linux-gnu"... WARNING: cannot read linux_banner string /proc/version: Linux version 2.6.16-xen0 (sgraham(a)anna.sn.stratus.com) (gcc version 3.4.4 20050721 (Red Hat 3.4.4-2)) #3 Sat May 6 19:04:09 EDT 2006 linux_banner: crash: /boot/vmlinux-syms-2.6.16-xen0 and /dev/mem do not match! Usage: crash [-h [opt]][-v][-s][-i file][-d num] [-S] [mapfile] [namelist] [dumpfile] Enter "crash -h" for details. ________________________________ -- Crash-utility mailing list Crash-utility(a)redhat.com https://www.redhat.com/mailman/listinfo/crash-utility