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

[Haozhong Zhang]

On Mon, Oct 05, 2015 at 09:53:26PM +0200, Radim KrÄmÃÅ 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@xxxxxxxxx>
> > ---
> > 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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