linux-kernel - Re: [PATCH 01/16] x86/tsc: Add a standalone helpers for getting TSC info from CPUID.0x15

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Message-ID: <Z6E9JyybI6SUWlcG@google.com>
Date: Mon, 3 Feb 2025 14:03:19 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Nikunj A Dadhania <nikunj@....com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, Juergen Gross <jgross@...e.com>, 
	"K. Y. Srinivasan" <kys@...rosoft.com>, Haiyang Zhang <haiyangz@...rosoft.com>, Wei Liu <wei.liu@...nel.org>, 
	Dexuan Cui <decui@...rosoft.com>, Ajay Kaher <ajay.kaher@...adcom.com>, 
	Alexey Makhalov <alexey.amakhalov@...adcom.com>, Jan Kiszka <jan.kiszka@...mens.com>, 
	Paolo Bonzini <pbonzini@...hat.com>, Andy Lutomirski <luto@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org, 
	linux-coco@...ts.linux.dev, virtualization@...ts.linux.dev, 
	linux-hyperv@...r.kernel.org, jailhouse-dev@...glegroups.com, 
	kvm@...r.kernel.org, xen-devel@...ts.xenproject.org, 
	Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH 01/16] x86/tsc: Add a standalone helpers for getting TSC
 info from CPUID.0x15

On Mon, Feb 03, 2025, Nikunj A Dadhania wrote:
> Sean Christopherson <seanjc@...gle.com> writes:
> > Extract retrieval of TSC frequency information from CPUID into standalone
> > helpers so that TDX guest support and kvmlock can reuse the logic.  Provide
> 
> s/kvmlock/kvmclock
> 
> > a version that includes the multiplier math as TDX in particular does NOT
> > want to use native_calibrate_tsc()'s fallback logic that derives the TSC
> > frequency based on CPUID.0x16 when the core crystal frequency isn't known.
> >
> > No functional change intended.
> >
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > ---
> 
> ...
> 
> > +
> > +static inline int cpuid_get_tsc_freq(unsigned int *tsc_khz,
> > +				     unsigned int *crystal_khz)
> 
> Should we add this in patch 6/16 where it is being used for the first time ?

No strong preference on my end.  I put it here mostly to keep each patch focused
on a single subsystem where possible, since the series touches so many areas.  I
also wanted to show the "full" API in a single patch, but I agree that adding a
helper without a user is generally undesirable.