Re: [Xen-devel] [PATCH 00/13] Add VMX TSC scaling support

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 11/24/2015 08:05 AM, Haozhong Zhang wrote:
> > >    This bug can be fixed either later by patch 5 which introduces a
> > >    common function hvm_scale_tsc() to scale TSC, or by replacing above
> > >    underlined code with a simplified and inlined version of
> > >    hvm_scale_tsc() as below:
> > >        uint64_t mult, frac;
> > >        mult    = ratio >> 32;
> > >        frac    = ratio & ((1ULL << 32) - 1);
> > >        offset  = host_tsc * mult;
> > >        offset += (host_tsc >> 32) * frac;
> > >        offset += ((host_tsc & ((1ULL << 32) - 1)) * frac) >> 32;
> > I am not sure I understand the last line (or maybe 2 lines)
> Just simple math with carefulness to avoid 64-bit integer overflow:
>
> suppose the most significant 32 bits of host_tsc and ratio are tsc_h
> and mult, and the least significant 32 bits of them are tsc_l and
> frac, then
>      host_tsc * ratio * 2^-32
>      = host_tsc * (mult * 2^32 + frac) * 2^-32
>      = host_tsc * mult + (tsc_h * 2^32 + tsc_l) * frac * 2^-32
>      = host_tsc * mult + tsc_h * frac + ((tsc_l * frac) >> 32)
Ok, now I see. Please include this in patch comments.

-boris

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