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Re: [PATCH v4 02/47] x86/tsc: Add a standalone helpers for getting TSC info from CPUID.0x15
- To: Borislav Petkov <bp@xxxxxxxxx>
- From: Sean Christopherson <seanjc@xxxxxxxxxx>
- Date: Tue, 9 Jun 2026 12:28:50 -0700
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- Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>, x86@xxxxxxxxxx, Kiryl Shutsemau <kas@xxxxxxxxxx>, "K. Y. Srinivasan" <kys@xxxxxxxxxxxxx>, Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>, Wei Liu <wei.liu@xxxxxxxxxx>, Dexuan Cui <decui@xxxxxxxxxxxxx>, Long Li <longli@xxxxxxxxxxxxx>, Ajay Kaher <ajay.kaher@xxxxxxxxxxxx>, Alexey Makhalov <alexey.makhalov@xxxxxxxxxxxx>, Jan Kiszka <jan.kiszka@xxxxxxxxxxx>, Andy Lutomirski <luto@xxxxxxxxxx>, Peter Zijlstra <peterz@xxxxxxxxxxxxx>, Juergen Gross <jgross@xxxxxxxx>, Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>, John Stultz <jstultz@xxxxxxxxxx>, "H. Peter Anvin" <hpa@xxxxxxxxx>, Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>, Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>, Broadcom internal kernel review list <bcm-kernel-feedback-list@xxxxxxxxxxxx>, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, Stephen Boyd <sboyd@xxxxxxxxxx>, kvm@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, linux-coco@xxxxxxxxxxxxxxx, linux-hyperv@xxxxxxxxxxxxxxx, virtualization@xxxxxxxxxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxxx, David Woodhouse <dwmw@xxxxxxxxxxxx>, Tom Lendacky <thomas.lendacky@xxxxxxx>, Nikunj A Dadhania <nikunj@xxxxxxx>, David Woodhouse <dwmw2@xxxxxxxxxxxxx>, Michael Kelley <mhklinux@xxxxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx>
- Delivery-date: Tue, 09 Jun 2026 19:28:57 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On Mon, Jun 01, 2026, Borislav Petkov wrote:
> On Fri, May 29, 2026 at 07:43:49AM -0700, Sean Christopherson wrote:
> > +static int cpuid_get_tsc_info(struct cpuid_tsc_info *info)
> > +{
> > + unsigned int ecx_hz, edx;
> > +
> > + memset(info, 0, sizeof(*info));
>
> Let's not clear this unnecessarily...
>
> > +
> > + if (boot_cpu_data.cpuid_level < CPUID_LEAF_TSC)
> > + return -ENOENT;
>
> ... just to return here...
>
> > +
> > + /* CPUID 15H TSC/Crystal ratio, plus optionally Crystal Hz */
> > + cpuid(CPUID_LEAF_TSC, &info->denominator, &info->numerator, &ecx_hz,
> > &edx);
> > +
> > + if (!info->denominator || !info->numerator)
> > + return -ENOENT;
>
> ... or here.
>
> We wanna clear it here, when we'll return success.
Actually, if we take the approach of relying on the user to check the return
code, then there's no need to zero the struct since all fields will be
explicitly
written, especially if we drop the "tsc_khz" field. I was zeroing the field
purely as defense in depth.
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