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Re: [PATCH 04/12] KVM: x86: Replace call-back set_tsc_khz() with a common function

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On Mon, Oct 05, 2015 at 09:53:26PM +0200, Radim Krčmář wrote:
> 2015-09-28 13:38+0800, Haozhong Zhang:
> > Both VMX and SVM propagate virtual_tsc_khz in the same way, so this
> > patch removes the call-back set_tsc_khz() and replaces it with a common
> > function.
> > 
> > Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
> > ---
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > +static void set_tsc_khz(struct kvm_vcpu *vcpu, u32 user_tsc_khz, bool scale)
> > +{
> > +	u64 ratio, khz;
> | [...]
> > +	khz = user_tsc_khz;
> 
> I'd use "user_tsc_khz" directly.
>

I'll do so.

> > +	/* TSC scaling required  - calculate ratio */
> > +	shift = (kvm_tsc_scaling_ratio_frac_bits <= 32) ?
> > +		kvm_tsc_scaling_ratio_frac_bits : 32;
> > +	ratio = khz << shift;
> > +	do_div(ratio, tsc_khz);
> > +	ratio <<= (kvm_tsc_scaling_ratio_frac_bits - shift);
> 
> VMX is losing 16 bits by this operation;  normal fixed point division
> could get us a smaller drift (and an one-liner here) ...
> at 4.3 GHz, 32 instead of 48 bits after decimal point translate to one
> "lost" TSC tick per second, in the worst case.
>
> Please mention that we are truncating on purpose :)

It's intentional to avoid the potential overflow in
  khz << kvm_tsc_scaling_ratio_frac_bits.

For VMX where kvm_tsc_scaling_ratio_frac_bits == 48, the above
expression is only safe to left shift a pretty small khz (< 2^16 KHz
or 65.5 MHz). Thus, I decided to sacrifice the precision for safety.
I chose to truncate at the boundary of 32 bits which can handle
khz as large as 4294 GHz.

Though this truncation results in losing TSC ticks when khz is larger
than 4.3 GHz, the lost is however pretty small compared with the large
khz.

Alternatively, it's also possible to follow David's comment to use
divq on x86_64 to keep both precision and safety. On i386, it just
falls back to above truncating approach.

- Haozhong
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[PATCH 00/12] KVM: x86: add support for VMX TSC scaling Haozhong Zhang <haozhong.zhang@intel.com> - 2015-09-28 07:40 +0200
  [PATCH 07/12] KVM: x86: Move TSC scaling logic out of call-back read_l1_tsc() Haozhong Zhang <haozhong.zhang@intel.com> - 2015-09-28 07:40 +0200
  [PATCH 11/12] KVM: VMX: Use a scaled host TSC for guest readings of MSR_IA32_TSC Haozhong Zhang <haozhong.zhang@intel.com> - 2015-09-28 07:40 +0200
  [PATCH 06/12] KVM: x86: Move TSC scaling logic out of call-back adjust_tsc_offset() Haozhong Zhang <haozhong.zhang@intel.com> - 2015-09-28 07:40 +0200
    Re: [PATCH 06/12] KVM: x86: Move TSC scaling logic out of call-back  adjust_tsc_offset() Paolo Bonzini <pbonzini@redhat.com> - 2015-09-28 22:20 +0200
      Re: [PATCH 06/12] KVM: x86: Move TSC scaling logic out of call-back  adjust_tsc_offset() Haozhong Zhang <haozhong.zhang@intel.com> - 2015-09-29 03:50 +0200
  [PATCH 01/12] KVM: x86: Collect information for setting TSC scaling ratio Haozhong Zhang <haozhong.zhang@intel.com> - 2015-09-28 07:40 +0200
    Re: [PATCH 01/12] KVM: x86: Collect information for setting TSC  scaling ratio Eric Northup <digitaleric@google.com> - 2015-09-29 05:30 +0200
      Re: [PATCH 01/12] KVM: x86: Collect information for setting TSC  scaling ratio Haozhong Zhang <haozhong.zhang@intel.com> - 2015-09-29 06:10 +0200
  [PATCH 02/12] KVM: x86: Add a common TSC scaling ratio field in kvm_vcpu_arch Haozhong Zhang <haozhong.zhang@intel.com> - 2015-09-28 07:50 +0200
    Re: [PATCH 02/12] KVM: x86: Add a common TSC scaling ratio field in  kvm_vcpu_arch Radim Krčmář <rkrcmar@redhat.com> - 2015-10-05 21:30 +0200
      Re: [PATCH 02/12] KVM: x86: Add a common TSC scaling ratio field in  kvm_vcpu_arch Haozhong Zhang <haozhong.zhang@intel.com> - 2015-10-06 04:00 +0200
  [PATCH 08/12] KVM: x86: Use the correct vcpu's TSC rate to compute time scale Haozhong Zhang <haozhong.zhang@intel.com> - 2015-09-28 07:50 +0200
    Re: [PATCH 08/12] KVM: x86: Use the correct vcpu's TSC rate to  compute time scale Radim Krčmář <rkrcmar@redhat.com> - 2015-10-05 22:20 +0200
  [PATCH 05/12] KVM: x86: Replace call-back compute_tsc_offset() with a common function Haozhong Zhang <haozhong.zhang@intel.com> - 2015-09-28 07:50 +0200
  [PATCH 03/12] KVM: x86: Add a common TSC scaling function Haozhong Zhang <haozhong.zhang@intel.com> - 2015-09-28 07:50 +0200
    Re: [PATCH 03/12] KVM: x86: Add a common TSC scaling function Paolo Bonzini <pbonzini@redhat.com> - 2015-09-28 22:20 +0200
      Re: [PATCH 03/12] KVM: x86: Add a common TSC scaling function Haozhong Zhang <haozhong.zhang@intel.com> - 2015-09-29 04:00 +0200
  [PATCH 10/12] KVM: VMX: Setup TSC scaling ratio when a vcpu is loaded Haozhong Zhang <haozhong.zhang@intel.com> - 2015-09-28 07:50 +0200
  [PATCH 04/12] KVM: x86: Replace call-back set_tsc_khz() with a common function Haozhong Zhang <haozhong.zhang@intel.com> - 2015-09-28 07:50 +0200
    Re: [PATCH 04/12] KVM: x86: Replace call-back set_tsc_khz() with a  common function Haozhong Zhang <haozhong.zhang@intel.com> - 2015-09-29 05:50 +0200
    Re: [PATCH 04/12] KVM: x86: Replace call-back set_tsc_khz() with a  common function Radim Krčmář <rkrcmar@redhat.com> - 2015-10-05 22:00 +0200
      Re: [PATCH 04/12] KVM: x86: Replace call-back set_tsc_khz() with a  common function David Matlack <dmatlack@google.com> - 2015-10-05 22:50 +0200
      Re: [PATCH 04/12] KVM: x86: Replace call-back set_tsc_khz() with a  common function Haozhong Zhang <haozhong.zhang@intel.com> - 2015-10-06 06:10 +0200
        Re: [PATCH 04/12] KVM: x86: Replace call-back set_tsc_khz() with a  common function Paolo Bonzini <pbonzini@redhat.com> - 2015-10-06 12:50 +0200
          Re: [PATCH 04/12] KVM: x86: Replace call-back set_tsc_khz() with a  common function Haozhong Zhang <haozhong.zhang@intel.com> - 2015-10-06 13:40 +0200
  Re: [PATCH 00/12] KVM: x86: add support for VMX TSC scaling Haozhong Zhang <haozhong.zhang@intel.com> - 2015-09-29 06:10 +0200
  Re: [PATCH 00/12] KVM: x86: add support for VMX TSC scaling Eric Northup <digitaleric@google.com> - 2015-09-29 06:10 +0200

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