Re: x86: kvm: Revert "remove sched notifier for cross-cpu migrations"

From: Andy Lutomirski <luto@amacapital.net>
To: Marcelo Tosatti <mtosatti@redhat.com>
Cc: kvm-devel <kvm@vger.kernel.org>, stable <stable@vger.kernel.org>,
	Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: x86: kvm: Revert "remove sched notifier for cross-cpu migrations"
Date: Mon, 23 Mar 2015 16:30:50 -0700	[thread overview]
Message-ID: <CALCETrULGgKaScwJHfyAJf4KYF5sve6u6T=HYv=aiRsdQJDO_Q@mail.gmail.com> (raw)
In-Reply-To: <20150323232151.GA12772@amt.cnet>

On Mon, Mar 23, 2015 at 4:21 PM, Marcelo Tosatti <mtosatti@redhat.com> wrote:
>
> The following point:
>
>     2. per-CPU pvclock time info is updated if the
>        underlying CPU changes.
>
> Is not true anymore since "KVM: x86: update pvclock area conditionally,
> on cpu migration".
>
> Add task migration notification back.

IMO this is a pretty big hammer to use to work around what appears to
be a bug in the host, but I guess that's okay.

It's also unfortunate in another regard: it seems non-obvious to me
how to use this without reading the cpu number twice in the vdso.  On
the other hand, unless we have a global pvti, or at least a global
indication of TSC stability, I don't see how to do that even with the
host bug fixed.

Grumble.

On a more useful note, could you rename migrate_count to
migrate_from_count, since that's what it is?

--Andy