Re: [PATCH] x86/amd: do not expose HWCR.TscFreqSel to guests

[Jan Beulich <jbeulich@suse.com>]

On 12.09.2023 18:23, Roger Pau Monne wrote:
> OpenBSD will attempt to unconditionally access PSTATE0 if HWCR.TscFreqSel is
> set, and will also attempt to unconditionally access HWCR if the TSC is
> reported as Invariant.
> 
> The reasoning for exposing HWCR.TscFreqSel was to avoid Linux from printing a
> (bogus) warning message, but doing so at the cost of OpenBSD not booting is not
> a suitable solution.

Why is the warning bogus? The PPR doesn't even state what the bit being
clear means; it's a r/o one. On respective families it cannot possibly
be correct to expose it clear.

> In order to fix expose an empty HWCR.
> 
> Fixes: 14b95b3b8546 ('x86/AMD: expose HWCR.TscFreqSel to guests')
> Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
> ---
> Not sure whether we want to expose something when is_cpufreq_controller() is
> true, seeing as there's a special wrmsr handler for the same MSR in that case.
> Likely should be done for PV only, but also likely quite bogus.

Well, keying to is_cpufreq_controller() is indeed questionable, but is
there any reason to not minimally expose the bit correctly when a
domain cannot migrate?

Jan