Re: [RFC PATCH v2 2/2] add support for Hyper-V invariant TSC

[Vadim Rozenfeld <vrozenfe@redhat.com>]

----- Original Message -----
From: "Marcelo Tosatti" <mtosatti@redhat.com>
To: "Gleb Natapov" <gleb@redhat.com>
Cc: "Peter Lieven" <pl@dlhnet.de>, "Vadim Rozenfeld" <vrozenfe@redhat.com>, kvm@vger.kernel.org, pl@dlh.net
Sent: Thursday, May 23, 2013 11:35:59 PM
Subject: Re: [RFC PATCH v2 2/2] add support for Hyper-V invariant TSC

On Thu, May 23, 2013 at 12:13:16PM +0300, Gleb Natapov wrote:
> > >
> > >"Reference TSC during Save and Restore and Migration
> > >
> > >To address migration scenarios to physical platforms that do not support
> > >iTSC, the TscSequence field is used. In the event that a guest partition
> > >is  migrated from an iTSC capable host to a non-iTSC capable host, the
> > >hypervisor sets TscSequence to the special value of 0xFFFFFFFF, which
> > >directs the guest operating system to fall back to a different clock
> > >source (for example, the virtual PM timer)."
> > >
> > >Why it would not/does not work after migration?
> > >
> > >
> > 
> > what exactly do we heed the reference TSC for? the reference counter alone works great and it seems
> > that there is a lot of trouble and crash possibilities involved with the referece tsc.
> > 
> Reference TSC is even faster. There should be no crashed with proper
> implementation.
> 
> --
> 			Gleb.

Lack of invariant TSC support in the host.

if there is no iTSC in the host -> set sequence to 0 and go with
reference counter. It is why they both scaled to 10 MHz, and it's
why reference counters is a fall-back for iTSC.