Re: [Xen-devel] Question about patch "svm: fix incorrect TSC scaling"

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 28.05.18 at 14:25, <dongli.zhang@xxxxxxxxxx> wrote:
> I am writing to ask about below patch as I am not familiar with AMD tsc
> scaling:
> 
>  commit 11eeca65126e51f03a883907751d5ccbe4f35aa3
>  Author: Haozhong Zhang <haozhong.zhang@xxxxxxxxx>
>  Date:   Tue Dec 8 09:46:12 2015 +0100
> 
>      svm: fix incorrect TSC scaling
>  
>      SVM TSC ratio is incorrectly used in the current
>      svm_get_tsc_offset(). This patch replaces the scaling logic in
>      svm_get_tsc_offset() with a correct implementation.
>  
>      Signed-off-by: Haozhong Zhang <haozhong.zhang@xxxxxxxxx>
>      Reviewed-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> 
> 
> The below equation is used in the above patch:
> 
> -------------------------
> host_tsc * ratio * 2^-32.
> -------------------------
> 
> 
> While the below incorrect equation (indeed code) was used before above 
> patch:
> 
> ---------------------------------------------------------------
> offset = (((host_tsc >> 32U) * (ratio >> 32U)) << 32U) +
>          (host_tsc & 0xffffffffULL) * (ratio & 0xffffffffULL);
> return guest_tsc - offset;
> ---------------------------------------------------------------
> 
> Can I summarize the above code as:
> 
> tsc_h * mult * 2^32 + tsc_l * frag or
> 
> tsc_h * int * 2^32 + tsc_l * frag ?
> 
> 
> Would you please help confirm why the equation is replaced? Or would you 
> please
> help and let me know the chapter/page of AMD reference manual showing the
> correct equation?

This has nothing to do with AMD's doc, but only with maths: To split
a multiplication x * y when considering x = xh + xl (high and low parts)
and y = yh + yl you get x * y = xh * yh + xh * yl + xl * yh + xl * xl.
Width constraints make it unnecessary to calculate the xh * yh part
here. The original expression, however, calculated something rather
strange instead (only the xl * yl part was actually correct).

Jan



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