Re: [Xen-devel] deleting mtrrs in dom0

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 24 Mar 2005, at 18:06, Kurt Garloff wrote:

> I seem to be undable to delete MTRRs in domain0:
>
> root@tpkurt:~ [0]# cat /proc/mtrr
> reg00: base=0x00000000 (   0MB), size= 512MB: write-back, count=0
> reg01: base=0x1ff80000 ( 511MB), size= 512KB: uncachable, count=0
> reg02: base=0xe0000000 (3584MB), size=  32MB: write-combining, count=0
> reg03: base=0xd0000000 (3328MB), size= 256MB: write-combining, count=0
> root@tpkurt:~ [0]# echo "disable=1" /proc/mtrr
> disable=1 /proc/mtrr
> root@tpkurt:~ [0]# echo "disable=1" >/proc/mtrr
> root@tpkurt:~ [1]# cat /proc/mtrr
> reg00: base=0x00000000 (   0MB), size= 512MB: write-back, count=0
> reg01: base=0x1ff80000 ( 511MB), size= 512KB: uncachable, count=0
> reg02: base=0xe0000000 (3584MB), size=  32MB: write-combining, count=0
> reg03: base=0xd0000000 (3328MB), size= 256MB: write-combining, count=0
>
> Isn't this supposed to work?
It's been hacked just enough to get X working, but the /proc semantics 
haven't been given much thought. The problem is that reading /proc/mtrr 
tells you what the real physical MTRRs contain, but these registers 
weren't initialised by domain0 but by Xen, during SMP bootstrap. So 
when you attempt to delete them you will probably find error messages 
in /var/log/messages saying 'reg1 has count=0'.
There are a few possible fixes, one being to let domain0 get staright 
at the MTRRs after it has booted, but this needs great care -- the 
registers have to be carefully synchronised across all physical CPUs to 
avoid memory deadlocks.
Better might be to have a low-level stateless interface exported by 
Xen, and have domain0 read the MTRR state during boot and set its 
refcnt to one for each MTRR that is initialised and in use.
 -- Keir



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