linux-kernel - Re: [PATCH RFC Linux] x86/xen: allow using x2APIC under Xen without CONFIG

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Date:	Tue, 01 Sep 2015 00:45:09 -0600
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Boris Ostrovsky" <boris.ostrovsky@...cle.com>
Cc:	"David Vrabel" <david.vrabel@...rix.com>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"xen-devel" <xen-devel@...ts.xenproject.org>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>,
	"Ingo Molnar" <mingo@...hat.com>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	"Alok Kataria" <akataria@...are.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH RFC Linux] x86/xen: allow using x2APIC under Xen
 without CONFIG_XEN enabled

>>> On 31.08.15 at 21:19, <boris.ostrovsky@...cle.com> wrote:
> On 08/20/2015 08:04 AM, Jan Beulich wrote:
>> While commit 4cca6ea04d31c claims to not have any functional effect on
>> Xen, this isn't the case: Before that change, kernels built without
>> CONFIG_XEN_PVHVM (a dependency which meanwhile became just CONFIG_XEN)
>> were able to run in x2APIC mode just fine. Restore that behavior.
>>
>> This, however, still doesn't fix the case where CONFIG_HYPERVISOR_GUEST
>> is not enabled, but I suppose this may be regarded as intentional.
>>
>> Signed-off-by: Jan Beulich<jbeulich@...e.com>
>> ---
>> The patch is RFC solely because the way the issue gets fixed doesn't
>> look very neat, but I couldn't figure out a better way.
> 
> (+ x86 maintainers)
> 
> Can we provide something like xen_stub.c (that will have its own 
> x86_hyper ops, probably only x2apic_available and maybe detect) which is 
> built when !CONFIG_XEN?
> 
> Otherwise I see little reason to keep x2apic_available op and we should 
> revert the portion of 4cca6ea04d31c that introduced it.

Actually I've considered that last option too, but it seemed no neater
than the presented patch (yet imo better than introducing a stub file).

Jan

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