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xen-devel
RE: [Xen-devel] [RFC] Physical hot-add cpus and TSC
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To: |
"Dong, Eddie" <eddie.dong@xxxxxxxxx>, "Jiang, Yunhong" <yunhong.jiang@xxxxxxxxx>, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>, "Xen-Devel (xen-devel@xxxxxxxxxxxxxxxxxxx)" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx> |
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Subject: |
RE: [Xen-devel] [RFC] Physical hot-add cpus and TSC |
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From: |
Dan Magenheimer <dan.magenheimer@xxxxxxxxxx> |
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Date: |
Thu, 10 Jun 2010 06:15:12 -0700 (PDT) |
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Thu, 10 Jun 2010 06:18:17 -0700 |
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> From: Dong, Eddie [mailto:eddie.dong@xxxxxxxxx]
>
> > Below is test result:
> > a) With the patch
> > Before hotadd:
> > (XEN) TSC marked as reliable, warp = 0 (count=3)
> > (XEN) No domains have emulated TSC
> > (XEN) TSC marked as reliable, warp = 0 (count=4)
> > (XEN) No domains have emulated TSC
> > (XEN) TSC marked as reliable, warp = 0 (count=5)
> > (XEN) No domains have emulated TSC
> > (XEN) TSC marked as reliable, warp = 0 (count=6)
> > (XEN) No domains have emulated TSC
> >
> > After add
> > (XEN) TSC marked as reliable, warp = 1669912421214 (count=15)
> > (XEN) No domains have emulated TSC
> > (XEN) TSC marked as reliable, warp = 1669912421214 (count=16)
> > (XEN) No domains have emulated TSC
>
> If the warp is fixed, at least for HVM, this can be solved by adjusting
> the TSC_OFFSET with the additional warp to make guest see warp=0 for
> TSC invariant case. Anything missed?
Hi Eddie --
Two things:
1) The TSC_OFFSET doesn't work for PV domains.
2) For HVM, it is very difficult to choose a precise TSC_OFFSET so
that it passes a warp test. If it doesn't pass a warp test,
upstream kernels will stop using tsc as a clocksource resulting
in a big performance loss... and some applications that use TSC
and are not TSC resilient that may have been working fine
for weeks may suddenly break due to an event (adding a physical
CPU to Xen) that neither the app nor its (guest) OS is able
to detect.
Dan
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