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

To: 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: Wed, 26 May 2010 22:26:10 +0100
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Thread-topic: [Xen-devel] [RFC] Physical hot-add cpus and TSC
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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.
>> 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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