Re: [Xen-devel] [PATCH v5 2/6] x86/hvm: Setup TSC scaling ratio

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.02.16 at 16:42, <haozhong.zhang@xxxxxxxxx> wrote:
> On 02/24/16 08:01, Jan Beulich wrote:
>> >>> On 23.02.16 at 03:05, <haozhong.zhang@xxxxxxxxx> wrote:
>> > +    /* ratio = (gtsc_khz << hvm_funcs.tsc_scaling.ratio_frac_bits) / 
>> > cpu_khz */
>> > +    asm (
>> > +        "shldq %2,%1,%0; salq %2,%1; divq %3"
>> > +        : "+&d" (dummy), "+&a" (ratio)
>> > +        : "c" (hvm_funcs.tsc_scaling.ratio_frac_bits),
>> > +          "rm" ((u64) cpu_khz) );
>> 
>> And this DIVQ can't possibly #DE, e.g. when gtsc_khz is much larger
>> than cpu_khz?
>>
> 
> Oops, it could. Following check should be added before asm():
>         /* the quotient is too large to fit in the integral part of TSC 
> scaling ratio */
>         if ( gtsc_khz / cpu_khz >
>              (hvm_funcs.tsc_scaling.max_ratio >> 
> hvm_funcs.tsc_scaling.ratio_frac_bits )
>             return 0;

Well, wouldn't that need to be >= then, since the division truncates?

Jan


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