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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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
not modify the value in
these bits until they are defined. This is done by doing a
“read-modify-write” 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".

~Andrew

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