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Re: [Xen-devel] [PATCH] pvcpuid: mask TSC invariant bit for various circumstances

  • To: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, "Xen-Devel (E-mail)" <xen-devel@xxxxxxxxxxxxxxxxxxx>
  • From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
  • Date: Tue, 27 Oct 2009 20:58:47 +0000
  • Cc:
  • Delivery-date: Tue, 27 Oct 2009 13:59:18 -0700
  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>
  • Thread-index: AcpXRcKaJR5Dnh2hRQecyHPEad+AqAAAoQp1
  • Thread-topic: [Xen-devel] [PATCH] pvcpuid: mask TSC invariant bit for various circumstances

On 27/10/2009 20:40, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:

> The hacky part is my attempt for
> the same cpuid bit (Invariant TSC) to have slightly
> different interpretations depending on whether it is
> physical (cpuid) or virtualized (pvcpuid)

How are you overloading it? According to your last patch you would only make
that bit visible to the guest when the TSC is invariant, and will remain
invariant (due to migration being disabled). That's within native semantics
of that bit, right?

> So are you suggesting that the best mechanism for
> "userland hypercalls" is special reserved cpuid leaves?

If you want to provide 'feature flags' and other such info to the guest, it
seems a pretty reasonable approach, since it already exists and that's what
it's intended for. There may be other things you want to do for which it is
not an appropriate solution, but everything so far seems handleable via it.

> Also, any comments on the meat of my last reply?

Looked pretty complicated to me. Can't say as I fully understand it.

 -- Keir

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