[Xen-devel] /dev/mem

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

Wouldn't it be more reasonable to have /dev/mem represent machine addresses 
only outside of the range of physical memory
assigned to a VM, and physical addresses inside this range? That way, more 
consistent behavior should be achieved to
native systems.

What I'm seeing is that accesses to /dev/mem offset 0x100000 get a failure in 
__direct_remap_pfn_range() (including, if
debugging is turned on, two log messages from the hypervisor). If these 
accesses went to kernel memory instead,
consumers of this interface would get exactly what they get on a native OS.

Further I would think that such map attempts shouldn't even make it to the 
hypervisor, thus avoiding the log messages
(which I started looking into only because I thought they might indicate a 
latent bug of some sort). This would,
however, require knowledge in the kernel which addresses (or address ranges) 
represent physical memory (rather than
potential I/O memory regions).

Finally, while following the involved code paths, I noted a check that is made 
in __ioremap, by calling
is_local_lowmem(). It would seem to me that calling this function just for the 
first address is insufficient - the range
specified may begin in non-lowmem but end in lowmem. Or it may begin in lowmem 
and end in non-lowmem, in which case I
would think that virt_to_page() wouldn't work correctly anymore (permitting, 
say, an erroneous dd command issued by root
to result in an oops or maybe worse things).

Jan

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