Re: [Xen-devel] [PATCH] x86: detect CMOS aliasing on ports other then 0x70/0x71

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 25/09/15 12:00, Jan Beulich wrote:
>>>> On 25.09.15 at 11:55, <andrew.cooper3@xxxxxxxxxx> wrote:
>> It is safe for the SMM handler to use CMOS if it returns the index 
>> register back to how it found it.  Furthermore, I am willing to bet that 
>> there real SMM handlers out there which do do this.
> So what options do you see then here? Don't do anything (drop the
> patch) seems the only one I can see not getting in conflict with SMI.
> Yet using the patch as is would not make anything less safe, it would
> just have the potential of not always making things as safe as they
> could be.

Given some orthogonal interaction with cmos-rtc-probe recently, I am
fairly sure that we can remove all real RTC interaction from Xen.

Dom0 is required to provide fine-grained time via XENPF_settime{32,64}
which gives an absolute time to seed the other domain wallclocks with. 
The RTC was assumed always to be in UTC, which allows full date and time
information to be derived.

It would certainly simplify things to leave all of the RTC in dom0's hand.

~Andrew

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