[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [Patch 4/4] x86/hpet: Don't clear reserved bits in the General Configuration Register
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 _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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