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

To: "Jiang, Yunhong" <yunhong.jiang@xxxxxxxxx>, Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, "Xen-Devel (xen-devel@xxxxxxxxxxxxxxxxxxx)" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] [RFC] Physical hot-add cpus and TSC
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Thu, 27 May 2010 07:57:45 +0100
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Thread-topic: [Xen-devel] [RFC] Physical hot-add cpus and TSC
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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)?

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.

Thoughts? I can implement this, or whatever we can (attempt to) agree on,
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.

 -- Keir

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