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

["Jiang, Yunhong" <yunhong.jiang@xxxxxxxxx>]

>-----Original Message-----
>From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
>[mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Keir Fraser
>Sent: Thursday, May 27, 2010 5:26 AM
>To: Dan Magenheimer; Xen-Devel (xen-devel@xxxxxxxxxxxxxxxxxxx); Ian Pratt
>Subject: Re: [Xen-devel] [RFC] Physical hot-add cpus and TSC
>
>On 26/05/2010 20:46, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx>
>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.

--jyh

>>> Also, one reason Intel pushed the CPU hotplug logic is for RAS, and
>>> offlining CPUs that throw errors, which can clearly be supported with
>>> no concerns over TSC sync.
>>
>> OK, then would you accept a patch that disables physical cpu-hot-add
>> (but not delete) unless enabled with a boot option, if the patch
>> includes sufficient commenting and dmesg to explain the ramifications?
>> Then in future if it turns out that TSC syncing is mostly always
>> handled by firmware (doubtful but possible), the default for the
>> boot option can be reversed.
>
>If the patch will be acked by the Intel authors of the cpu hotplug stuff
>then yes.
>
> -- Keir
>
>
>
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