[Xen-devel] Re: [PATCH 02/02] Kexec / Kdump: Don't declare _end

[Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>]

Hey Magnus, 

On Mon, 2006-12-04 at 13:35 +0900, Magnus Damm wrote:
> [PATCH 02/02] Kexec / Kdump: Don't declare _end
> 
> _end is already declared in xen/include/asm/config.h, so don't declare 
> it twice. This solves a powerpc/ia64 build problem where _end is declared 
> as char _end[] compared to unsigned long _end on x86.

This change has broken x86 kdump :-( I think because you fixed a bug
with your change and thereby uncovered an another latent bug.

Before the range->size returned from kexec_get_xen() was 1/4 of the
correct value because you were subtracting unsigned long * pointers so
size was the number of words not the number of bytes as expected. After
this change we are now subtracting unsigned longs so the correct value
is returned.

This seems to have caused the crash notes to disappear from /proc/iomem:
        Before:
                00100000-def7efff : System RAM
                  00100000-001397bf : Hypervisor code and data
                  00193000-001930f7 : Crash note
                  00194000-001940f7 : Crash note
                  02000000-05ffffff : Crash kernel
        After:
                00100000-def7efff : System RAM
                  00100000-001e5eff : Hypervisor code and data
                  02000000-05ffffff : Crash kernel

I presume they went missing because "Hypervisor code and data" now
overlaps the notes.

For some reason this has broken kdump for me (on x86_32p). The kdump
kernel gives this stack trace and then hangs a little later on:
        general protection fault: 0000 [#1]
        Modules linked in:
        CPU:    0
        EIP:    0060:[<c204954d>]    Not tainted VLI
        EFLAGS: 00010002   (2.6.16.33-x86_32p-kdump #17)
        EIP is at free_block+0x6d/0x100
        eax: 00000000   ebx: ffffffff   ecx: ffffffff   edx: c2455000
        esi: 00000001   edi: c5f22540   ebp: c253bef0   esp: c253bed8
        ds: 007b   es: 007b   ss: 0068
        Process events/0 (pid: 4, threadinfo=c253a000 task=c5f0aa70)
        Stack: <0>00000001 c5e71210 00000000 c5e71210 00000001 c5e71200 
c253bf14 c2049625
               00000000 c234b100 c5f0aa70 c5f22540 c5f22588 c5f22540 c257a4c0 
c253bf34
               c204a796 00000000 00000086 00000000 c242d364 c5f51680 00000296 
c253bf64
        Call Trace:
         [<c2003685>] show_stack_log_lvl+0xc5/0xf0
         [<c2003847>] show_registers+0x197/0x220
         [<c20039ae>] die+0xde/0x210
         [<c20048fe>] do_general_protection+0xee/0x1a0
         [<c200310f>] error_code+0x4f/0x54
         [<c2049625>] drain_array_locked+0x45/0xa0
         [<c204a796>] cache_reap+0x66/0x130
         [<c2021456>] run_workqueue+0x66/0xd0
         [<c2021a08>] worker_thread+0x138/0x160
         [<c202461f>] kthread+0xaf/0xe0
         [<c2001005>] kernel_thread_helper+0x5/0x10

I changed xen_machine_kexec_register_resources() on the Linux side to
correctly nest the crash note resources under the xen resource which has
fixed things for me. Does the change below make sense to you? If so I'll
commit.

As a secondary point, perhaps the hypervisor resource should go all the
way to the end of the Xen heap (xenheap_phys_end I think) rather than
just the the end of .data/.bss?

Ian.

diff -r a0da9727b51d linux-2.6-xen-sparse/drivers/xen/core/machine_kexec.c
--- a/linux-2.6-xen-sparse/drivers/xen/core/machine_kexec.c     Mon Dec 04 
09:55:06 2006 +0000
+++ b/linux-2.6-xen-sparse/drivers/xen/core/machine_kexec.c     Mon Dec 04 
17:32:35 2006 +0000
@@ -106,7 +106,7 @@ void xen_machine_kexec_register_resource
        request_resource(res, &xen_hypervisor_res);
 
        for (k = 0; k < xen_max_nr_phys_cpus; k++)
-               request_resource(res, xen_phys_cpus + k);
+               request_resource(&xen_hypervisor_res, xen_phys_cpus + k);
 
 }
 



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