[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] 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 (xen-devel@xxxxxxxxxxxxxxxxxxx); >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 not support invariant TSC, I think current redezvous calibration code has do that already, right? > >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 get the TSC through ITP, that's not so convenient. > > -- Keir > _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel
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