Re: [Xen-devel] [PATCH v8] new config option vtsc_tolerance_khz to avoid TSC emulation

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Sun, Apr 01, 2018 at 10:29:58PM +0200, Olaf Hering wrote:
> Add an option to control when vTSC emulation will be activated for a
> domU with tsc_mode=default. Without such option each TSC access from
> domU will be emulated, which causes a significant perfomance drop for
> workloads that make use of rdtsc.
> 
> One option to avoid the TSC option is to run domUs with tsc_mode=native.
> This has the drawback that migrating a domU from a "2.3GHz" class host
> to a "2.4GHz" class host may change the rate at wich the TSC counter
> increases, the domU may not be prepared for that.
> 
> With the new option the host admin can decide how a domU should behave
> when it is migrated across systems of the same class. Since there is
> always some jitter when Xen calibrates the cpu_khz value, all hosts of
> the same class will most likely have slightly different values. As a
> result vTSC emulation is unavoidable. Data collected during the incident
> which triggered this change showed a jitter of up to 200 KHz across
> systems of the same class.
> 
> Existing padding fields are reused to store vtsc_khz_tolerance as u16.
> 
> Signed-off-by: Olaf Hering <olaf@xxxxxxxxx>

Reviewed-by: Wei Liu <wei.liu2@xxxxxxxxxx>

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