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Re: [Xen-devel] HPET enabled in BIOS, not presented as available_clocksource -- config, kernel code, &/or BIOS?

(apologies re: the empty 'double tap' :-/ )

On 5/14/17 8:39 AM, Andrew Cooper wrote:
>> So I'm still unclear -- is this^ now, correctly "all" using MSI/HPET?
> What are you trying to achieve?  It is still not clear despite all on
> this thread.
> The Linux HEPT error messages are non-ideal, but there is no way dom0
> will ever be able to use clocksource=hpet when running under Xen.

What I'm trying to achieve is to

        (a) understand, in general
        (b) correctly implement HPET usage in Xen
        (c) understand &, as needed, remediate the warnings/error message that 
seem(ed) to be associated

I.e. -- what exactly needs be done, and what should be the observable results, 
when "using" HPET with Xen.  It's simply not obvious from the docs.

The docs here,


are ... somewhat challenging:

        "By far Xen3.4 supports PIT/HPET as the broadcast source.
        HPET as broadcast timer source (clocksource) =
        HPET can delivery timely wakeup event to CPUs sleep in deep C-states 
with negligible overhead, as stated earlier. But HPET mode being used does make 
some differences to worthy of our noting:

            If h/w supports per-channel MSI delivery mode (intr via FSB), it's 
the best broadcast mechanism known so far.


OTOH, this comment:

On 5/15/17 11:06 AM, Austin S. Hemmelgarn wrote:
> That depends on what you mean by everything correctly using the HPET. 
> Using clocksource=xen (or autoselecting it) will cause the kernel to get 
> timing info from Xen.  If you're running as a guest, this is absolutely 
> what you want (unless you're using HVM), and with possible limited and 
> extremely specific exceptions, this is almost certainly what you want in 
> Domain-0 as well.  Given that Xen is using the HPEt for timing itself, 
> using clocksource=xen will result in Linux _indirectly_ using the HPET 
> through Xen without involving the HPET driver (in essence, Xen is your 
> HPET driver in this situation), which will get you essentially the same 
> precision that you would get by using the HPET directly.
> So, if you just want the precision offered by the HPET, then yes, you 
> are getting the same thing through the Xen clocksource.

Is legible, understandable & helpfully informative. (Thanks, Austin! Valentin's 
comments helped as well.)

'tho further detail on common &/or "limited and extremely specific exceptions" 
use-cases of PVH, HVM, PVHVM & HVM2 will be useful, I'd heartily recommend that 
some version of Austin's comment be added to the docs/wiki as a nice doc-step 

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