Re: KVM "Hyper-V emulation" -- this can be related "Linux on Hyper-V" ? Or "KVM emulates Hyper-V" as "Hyper-V emulates Hyper-V" ? Re: [PATCH 2/3] X86: Add a check to catch Xen emulation of Hyper-V

[Victor Miasnikov]

Hi!

2013/4/17 Jan Beulich
> >>> On 17.04.13 at 12:16, "Michael S. Tsirkin"  wrote:
> > If the hypervisor says it's Hyper-V, that's because it wants
> > guests to use Hyper-V. I don't see why is guest second-guessing
> > this a good idea.
>
> There are two reasons here: For one, when the hypervisor is not
> Hyper-V, but is providing some Hyper-V emulation, that's intended
> for Windows guests to use, not e.g. Linux ones, especially when
> such guests could use the native hypervisor interface with much
> greater benefit.
>
> And second, there reportedly are features of (newer?) Hyper-V
> that some emulation may not provide, but that are also not easily
> detectable.


Yes

+



> On Wed, Apr 17, 2013 at 12:10:01PM +0300, Victor Miasnikov wrote:



> > Question is very simple:  Hyper-V users/sysadmins need wait patch a-la this:
> > Or  "KVM_and_or_XEN_or_other_Hypervisor_what_has_non-full-emulation_of_Hyper-V emulates Hyper-V" as "Hyper-V emulates
> > Hyper-V" ?
>
> No.  You are using Hyper-V, not the KVM_and_or_XEN_or_other_Hypervisor_what_has_non-full-emulation_of_Hyper-V
> emulation of it.
> No patches dealing with this emulation should have any effect on you.


This is _positive_ variant of possible future,
but on _practic_ very often we need prevent _negative_ variant of possible future


> > ==
> > + /*
> > +  * KVM_and_or_XEN_or_other_Hypervisor_what_has_non-full-emulation_of_Hyper-V emulates Hyper-V to support
> > enlightened Windows.
> > +  * Check to see first if we are on a KVM_and_or_XEN_or_other_Hypervisor_what_has_non-full-emulation_of_Hyper-V
> > Hypervisor.
> > +  */
> > +
> > + if (KVM_and_or_XEN_or_other_Hypervisor_what_has_non-full-emulation_of_Hyper-V_cpuid_base())
> > +  return false;

> If the hypervisor says it's Hyper-V, that's because it wants guests to use Hyper-V.


Even if guest ( i.e. Virtual Computer) contain Linux OS ?


> > ==
> > KVM_and_or_XEN_or_other_Hypervisor_what_has_non-full-emulation_of_Hyper-V emulates Hyper-V to host enlightened
> > Windows.
> >
> > . . .
> >
> > [ hpa: the problem here is that
> > KVM_and_or_XEN_or_other_Hypervisor_what_has_non-full-emulation_of_Hyper-V doesn't emulate Hyper-V well enough,  .
> >  . ]
>
> What's emulated not well enough?

In case of XEN variant non-full-emulation_of_Hyper-V , IMHO :

{{
----- Original Message ----- 
From: "Victor Miasnikov"
To: "Michael S. Tsirkin"
Sent: Wednesday, April 17, 2013 12:41 PM
Subject: IMHO, XEN emulation of Hyper-V not contain needed clocksource for Lunux guest Re: [PATCH 2/3] X86: Add a check
to catch Xen emulation of Hyper-V

Hi!

> It seems that one might want to use hyper-v emulation e.g. to test
> hyper-v code without using windows, so the functionality
> that this patch disables is not completely useless,
> so there should be a good reason for disabling it.

As I undestand, XEN emulation of Hyper-V not contain  needed clocksource for Lunux guest ( virtual computers)

I.e. Linux VM "think" what is running on Hyper-V host call clocksource_register_hz(&hyperv_cs, NSEC_PER_SEC/100)
and a-la BSOD or vice versa . . .


> I went over the original discussion in
> https://patchwork.kernel.org/patch/2064331/
> and that's still not clear to me. Is there a configuration
> that is broken without this patch but starts working with
> this patch?


IMHO, this related to:

- clocksource_register_hz(&hyperv_cs, NSEC_PER_SEC/100);
+ if (ms_hyperv.features & HV_X64_MSR_TIME_REF_COUNT_AVAILABLE)
+  clocksource_register_hz(&hyperv_cs, NSEC_PER_SEC/100);


See

https://lkml.org/lkml/2013/2/12/623
==
           Date Tue, 12 Feb 2013 16:56:07 -0800
           From tip-bot for Olaf Hering
           Subject [tip:x86/hyperv] x86: Hyper-V: register clocksource only if its advertised

. . .

==



Best regards, Victor Miasnikov
}}





Best regards, Victor Miasnikov
Blog:  http://vvm.blog.tut.by/

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