Re: [PATCH v16 12/13] x86/tsc: Switch to native sched clock

[Sean Christopherson <seanjc@google.com>]

On Wed, Jan 08, 2025, Borislav Petkov wrote:
> On Wed, Jan 08, 2025 at 06:00:59AM -0800, Sean Christopherson wrote:
> > Ideally, if the TSC is the preferred clocksource, then the scheduler will use the
> > TSC and not a PV clock irrespective of STSC.  But I 100% agree with Boris that
> > it needs buy-in from other maintainers (including Paolo), because it's entirely
> > possible (likely, even) that there's an angle to scheduling I'm not considering.
> 
> That's exactly why I wanted to have this taken care of only for the STSC side
> of things now and temporarily. So that we can finally land those STSC patches
> - they've been pending for waaay too long.
> 
> And then ask Nikunj nicely to clean up this whole pv clock gunk, potentially
> kill some of those old clocksources which probably don't matter anymore.
> 
> But your call how/when you wanna do this.

I'm okay starting with just TDX and SNP guests, but I don't want to special case
SNP's Secure TSC anywhere in kvmclock or common TSC/sched code.

For TDX guests, the TSC is _always_ "secure".  So similar to singling out kvmclock,
handling SNP's STSC but not the TDX case again leaves the kernel in an inconsistent
state.  Which is why I originally suggested[*] fixing the sched_clock mess in a
generically; doing so would avoid the need to special case SNP or TDX in code
that doesn't/shouldn't care about SNP or TDX.

[*] https://lore.kernel.org/all/ZurCbP7MesWXQbqZ@google.com

> If you want the cleanup first, I'll take only a subset of the STSC set so that
> I can unload some of that set upstream.

My vote is to apply through "x86/sev: Mark Secure TSC as reliable clocksource",
and then split "x86/tsc: Switch Secure TSC guests away from kvm-clock" to grab
only the snp_secure_tsc_init() related changes (which is how that patch should
be constructed no matter what; adding support for MSR_AMD64_GUEST_TSC_FREQ has
nothing to do with kvmclock).

And then figure out how to wrangle clocksource and sched_clock in a way that is
sane and consistent.