linux-kernel - Re: [patch 14/21] Xen-paravirt: Add XEN config options and disable unsupported config options.

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Date:	Fri, 16 Feb 2007 10:49:10 +0000
From:	Keir Fraser <keir@...source.com>
To:	Dan Hecht <dhecht@...are.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Chris Wright <chrisw@...s-sol.org>, Andi Kleen <ak@....de>,
	<xen-devel@...ts.xensource.com>,
	Ian Pratt <ian.pratt@...source.com>,
	<virtualization@...ts.osdl.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [patch 14/21] Xen-paravirt: Add XEN config options and
 disable unsupported config options.

On 16/2/07 08:37, "Dan Hecht" <dhecht@...are.com> wrote:

> Hmm?  I thought the periodic timer and one-shot timer both generate the
> same VIRQ.  So, how can you mask one without masking the other?
> 
> The tickless idle works since the block hypercall disables the periodic
> timer.  But for dynticks (aka NO_HZ), you'll need to mask the periodic
> timer (even for a running vcpu), while keeping the one-shot timer
> unmasked.  I don't think Xen provides an interface to do that.

There's no interface for this right now, but we can easily add one in a
backward compatible way. This also will not prevent a dyntick kernel from
running on an older Xen -- the guest will then receive to many timer
interrupts, but that should only be a performance concern fixable by
upgrading the hypervisor component of the system.

 -- Keir

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