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

[Gleb Natapov]

On Tue, Jun 30, 2009 at 11:43:41AM -0700, Yinghai Lu wrote:
> 2009/6/30 Gleb Natapov <gleb@xxxxxxxxxx>:
> 
> >>
> >> 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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