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xen-devel
Re: [Xen-devel] deleting mtrrs in dom0
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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