but

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
---
> CONFIG_CRASH=m
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

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