Re: [Xen-devel] [PATCH RFC] x86/lapic: remove the PIT usage to calibrate the lapic timer

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Wed, Sep 19, 2018 at 06:25:54PM +0200, Roger Pau Monne wrote:
> And instead use NOW which is based on the TSC. This was already used
> when running in shim mode, since there's likely no PIT in that
> environment.
> 
> Remove printing the CPU frequency, since it's already printed earlier
> at boot, and getting the CPU frequency against the TSC without any
> external reference timer is pointless.
> 
> The motivation behind this change is to allow Xen to boot on HyperV
> gen2 instances, which lack a PIT.

lacks

> 
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> ---
> I'm not sure about the reason behind the usage of the PIT instead of
> the TSC, maybe this was done because the TSC wasn't available until
> the Pentium? Xen certainly doesn't care about such hardware anymore,
> and the TSC is already used unconditionally in Xen.
> 
> Linux seems to prefer to calibrate the lapic timer against the PM
> timer and has already dropped PIT usage for that.
> 
> My early tests on a single box show no differences between the TSC or
> the PIT for lapic timer calibration, but that's a single box.
> 
> The RFC is because I'm not sure I understand the motivation for using
> the PIT in the first place, so I might be missing something relevant
> that could make this patch moot.

I wondered the same.

Wei.

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