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Re: [Xen-users] Xen Clocksource, the VDSO)... not how, but why... and some micro-benchmarks.

  • To: "xen-users@xxxxxxxxxxxxx" <xen-users@xxxxxxxxxxxxx>
  • From: Hans van Kranenburg <Hans.van.Kranenburg@xxxxxxxxxx>
  • Date: Mon, 11 Mar 2019 00:05:12 +0000
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  • Delivery-date: Mon, 11 Mar 2019 00:06:40 +0000
  • List-id: Xen user discussion <xen-users.lists.xenproject.org>
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  • Thread-topic: [Xen-users] Xen Clocksource, the VDSO)... not how, but why... and some micro-benchmarks.

On 3/7/19 5:39 PM, Hans van Kranenburg wrote:
> [...]
> I still haven't found the exact reason why we should add clocksource=tsc
> tsc=stable:socket to the hypervisor command line. It's not needed to
> make all tsc vdso trickery in the domU work. Moreover, it does not even
> seem needed at all to set clocksource=tsc in xen to be able to use it in
> the domU?
> The only place where I can find tsc=stable:socket being mentioned is
> https://lore.kernel.org/patchwork/cover/849340/ which should be a
> changeset to enable vdso calls for the xen clocksource (in linux 4.15).
> But that never got merged.

Well, I went on a journey to find out what tsc=stable:socket is actually
doing, and that journey ended rather soon. :D

The tsc=stable:socket was introduced in...

commit bc900cbc8f37b93cc6c9f6370beb14e6430b334d
Author: Joao Martins <joao.m.martins@xxxxxxxxxx>
Date:   Fri Sep 23 18:26:19 2016 +0200

    x86/time: extend "tsc" param with "stable:socket"

...and nothing at all seems to be using TSC_RELIABLE_SOCKET or the
tsc_flags introduced there in the code further on.

+/* TSC is reliable across sockets */
+#define TSC_RELIABLE_SOCKET (1 << 0)

So, that command line part is effectively a noop. Maybe it was added
because that linux patchset for xen clocksource wanted to use it? But,
those patches also don't explain what would be special about this extra
option and what it would be used for.

Hans van Kranenburg
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