linux-kernel - Re: [PATCH v4] enable x2APIC without interrupt remapping under KVM

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Message-ID: <20090630185347.GD8122@redhat.com>
Date:	Tue, 30 Jun 2009 21:53:48 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	Yinghai Lu <yhlu.kernel@...il.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Vivek Goyal <vgoyal@...hat.com>, linux-kernel@...r.kernel.org,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Sheng Yang <sheng@...ux.intel.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"avi@...hat.com" <avi@...hat.com>
Subject: Re: [PATCH v4] enable x2APIC without interrupt remapping under KVM

On Tue, Jun 30, 2009 at 11:43:41AM -0700, Yinghai Lu wrote:
> 2009/6/30 Gleb Natapov <gleb@...hat.com>:
> 
> >>
> >> how about kexec second kernel in KVM ?
> >>
> >> x2apic_preenabled will be set in second kernel.
> >>
> > Yes, bummer. But the similar problem exist now and without KVM. After
> > running x2apic enabled kernel you can't kexec kernel without x2apic
> > support.
> 
> if the first kernel enable x2apic and intr_remapping, and second
> kernel has x2apic and intr_remap support.
> we can kexec the second kernel.
> 
And if it hasn't we cannot, that is what I am trying to point out.

> restoring to original state is not possible for kdump case.
> 
Shouldn't it be enough to run UP kernel for kdump? So kdump can reset BSP
apic to xAPIC mode before starting a kernel with maxcpus=1.

> maybe with minor change to your patch, you could kexec the in kexec
> second kernel with x2apic support.
> 
My patch can easily avoid the issue by checking max_physical_apicid > 255
instead of x2apic_preenabled.

--
			Gleb.
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