linux-kernel - Re: [PATCH 02/10] KVM: x86: Improve accuracy of KVM clock when TSC scaling is in force

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Message-ID: <CABgObfa0j34iEh81hhd7-t7ZM1GKAsvJb5xP6EoD2-c-8TnPqQ@mail.gmail.com>
Date: Mon, 22 Apr 2024 14:22:54 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: David Woodhouse <dwmw2@...radead.org>
Cc: kvm@...r.kernel.org, Jonathan Corbet <corbet@....net>, 
	Sean Christopherson <seanjc@...gle.com>, 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, 
	"H. Peter Anvin" <hpa@...or.com>, Paul Durrant <paul@....org>, Shuah Khan <shuah@...nel.org>, 
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, Oliver Upton <oliver.upton@...ux.dev>, 
	Marcelo Tosatti <mtosatti@...hat.com>, jalliste@...zon.co.uk, sveith@...zon.de
Subject: Re: [PATCH 02/10] KVM: x86: Improve accuracy of KVM clock when TSC
 scaling is in force

On Thu, Apr 18, 2024 at 9:51 PM David Woodhouse <dwmw2@...radead.org> wrote:
>         gpa_t time;
>         struct pvclock_vcpu_time_info hv_clock;
> -       unsigned int hw_tsc_khz;
> +       unsigned int hw_tsc_hz;

Why not change this to u64? 4.3 GHz is scarily close to current
processors, though I expect that it will break a lot more software
than just KVM.

>  static int kvm_guest_time_update(struct kvm_vcpu *v)
>  {
> -       unsigned long flags, tgt_tsc_khz;
> +       unsigned long flags;
> +       uint64_t tgt_tsc_hz;

.. especially considering that you did use a 64-bit integer here
(though---please use u64 not uint64_t; and BTW if you want to add a
patch to change kvm_get_time_scale() to u64, please do.

Thanks,

Paolo