Re: [PATCH][4.15] x86/HPET: don't enable legacy replacement mode unconditionally

[Tamas K Lengyel <tamas.k.lengyel@xxxxxxxxx>]

On Wed, Mar 24, 2021 at 6:34 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>
> Commit e1de4c196a2e ("x86/timer: Fix boot on Intel systems using ITSSPRC
> static PIT clock gating") was reported to cause boot failures on certain
> AMD Ryzen systems. Until we can figure out what the actual issue there
> is, skip this new part of HPET setup by default. Introduce a "hpet"
> command line option to allow enabling this on hardware where it's really
> needed for Xen to boot successfully (i.e. where the PIT doesn't drive
> the timer interrupt).
>
> Since it makes little sense to introduce just "hpet=legacy-replacement",
> also allow for a boolean argument as well as "broadcast" to replace the
> separate "hpetbroadcast" option.

While having the command line option to control it is fine what would
really be the best solution is if Xen could figure out when the
legacy-replacement option is necessary to begin with and enable it on
its own, even if it's done as a fallback route. We'll have issues with
telling users when the option is needed and when it isn't. I don't
like the idea of users having to go through a route of "well, let's
see if Xen boots and if you get this weird crash/reboot add this
obscure boot option". It's just a bad user experience all around.

Tamas