[Question] About scaling factor of Enhanced Counter Virtualization

[zhukeqian]

Hi Marc,

On 2020/7/14 19:34, Marc Zyngier wrote:
> Hi Keqian,
> 
> On 2020-07-14 03:20, zhukeqian wrote:
>> Hi all,
>>
>> We are studying Enhanced Counter Virtualization (introduced by ARMv8.6
>> extension), and here is a question
>> raised by Biaoxiang Ye <yebiaoxiang at huawei.com>:
>>
>> Described in the ARMv8.6 Extension Specification:
>> Note: the scaling factor CNTSCALE is designed as a 2.62 bit fixed
>> point number, so permitting a scaling up by
>> (nearly) a factor 4. The scaling factor CNTISCALE is signed as an 8.56
>> number for the scaling of the values
>> written into the timers for comparison with the actual count. This
>> implies that the greatest scaling down of the
>> counter supported in (nearly) a factor of *512*.
>>
>> We think the number "512" should be "256" (2^8), or do we miss something?
> 
> This register doesn't seem to be described in ARM DDI 0487F.b,
> which is the official documentation and does contain the ARMv8.6
> material, including ARMv8.6-ECV.
> 
> Either you are looking at confidential information (and nobody
> can answer you in public), or obsolete information (and nobody
> knows what this is about).
> 
Well, it's alpha release version :( . Many thanks!

Thanks,
Keqian
> In any case, this is most probably a question for ARM's support
> team, and not the kernel mailing lists.
> 
> Thanks,
> 
>         M.