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RE: [Xen-devel] [RFC] Physical hot-add cpus and TSC

>-----Original Message-----
>From: Jiang, Yunhong
>Sent: Thursday, May 27, 2010 3:10 PM
>To: Keir Fraser; Dan Magenheimer; Xen-Devel (xen-devel@xxxxxxxxxxxxxxxxxxx); 
>Subject: RE: [Xen-devel] [RFC] Physical hot-add cpus and TSC
>>-----Original Message-----
>>From: Keir Fraser [mailto:keir.fraser@xxxxxxxxxxxxx]
>>Sent: Thursday, May 27, 2010 2:58 PM
>>To: Jiang, Yunhong; Dan Magenheimer; Xen-Devel
>>Ian Pratt
>>Subject: Re: [Xen-devel] [RFC] Physical hot-add cpus and TSC
>>On 27/05/2010 07:15, "Jiang, Yunhong" <yunhong.jiang@xxxxxxxxx> wrote:
>>>>>> It depends how physical CPU hotplug is implemented doesn't it. I expect
>>>>>> there's sufficient firmware involved in such an operation that TSCs
>>>>>> could
>>>>>> get synced up before host software gets a look in. I don't think we can
>>>>>> comment on whether or not there is an issue here without more
>>>>>> information.
>>> Yes, this is a issue. The TSC will not be synched by firmware when 
>>> hot-added,
>>> at least I didn't find any spec on this, and my experiment shows the TSC 
>>> value
>>> is very small when new CPU is brought up. We need sync it in Xen side,
>>> Is it possible to sync the new-added CPU with the BSP when the CPU is added
>>> (changed from non-present to present), as Keir suggested in previous mail? I
>>> will have a look on the related code.
>>Is this *specifically* a problem for physical cpu hot-add, but not 'logical'
>>cpu online (i.e, XENPF_cpu_hotadd but not XENPF_cpu_online)?
>Yes, I do think so, if the CPU support invariant TSC. For those CPU that does 
>support invariant TSC, I think current redezvous calibration code has do that 
>>We could sync an AP's TSC with the master CPU bringing it up (typically
>>CPU0) if (a) !boot_cpu_has(RELIABLE_TSC); or (b) The slave was introduced
>>via XENPF_cpu_hotadd and this is its first time brought online.
>Yes, exactly.
>>Thoughts? I can implement this, or whatever we can (attempt to) agree on,
>It's great if you can do that. I'm still checking the time related code.
>>easily enough. I expect Dan would prefer to have XENPF_cpu_hotadd disabled,
>>or RELIABLE_TSC disabled, depending on a command-line option defaulting to
>>the former. It seems a bit onerous to me however.
>Yes, thanks.
>BTW, are there any easy way to check the TSC skew in the system? Originally I 
>the TSC through ITP, that's not so convenient.

Just found the "t" debug key. Will try it.


>> -- Keir

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