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Re: [Xen-devel] [Patch 4/4] x86/hpet: Don't clear reserved bits in the General Configuration Register

>>> On 07.10.13 at 16:02, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
> On 07/10/13 14:55, Jan Beulich wrote:
>>>>> On 07.10.13 at 15:26, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
>>> It is a violation of the specification.
>>> The reserved bits in the General Configuration Register, unlike all other
>>> reserved bits I have found in the spec, are specified as 'must never be
>>> changed by the OS'.
>> Mind pointing out where exactly you found this? I only find the
>> usual "should not modify" statements, and it is really unclear
>> whether leaving the bits alone is more compatible than clearing
>> them (since a bit of unknown function being set may easily mean
>> the HPET behaves in a way we don't expect).
>> Jan
> Hpet spec 1-0a.pdf Page 12
> "General Configuration Register Bit Definitions"
> For bits 63:2, (ignoring the spec reserved vs firmware reserved bits),
> the requirement states:
> "In order to preserve usage of these bits in the future, software should

"should" != "must never"

> not modify the value in
> these bits until they are defined. This is done by doing a
> âread-modify-writeâ to this
> register."
> In most cases Xen does correctly perform a read-modify-write, but not on
> initialize examination of the hpet where it blindly tries to clear bits
> it doesn't understand.
> I did find it strange at the difference in the spec; All other reserved
> bits I can find are specified as "must write 0".

Right. As said before - it's all but clear whether leaving the bits
alone is indeed better than clearing them.


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