[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 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. Jan _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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