linux-kernel - Re: [PATCH v1 06/11] KVM: x86: use hardware-compatible format for APIC ID register

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Message-ID: <20160701155349.GB5965@potion>
Date:	Fri, 1 Jul 2016 17:53:50 +0200
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	"Lan, Tianyu" <tianyu.lan@...el.com>,
	Igor Mammedov <imammedo@...hat.com>,
	Jan Kiszka <jan.kiszka@....de>, Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH v1 06/11] KVM: x86: use hardware-compatible format for
 APIC ID register

2016-07-01 17:07+0200, Paolo Bonzini:
> On 01/07/2016 16:54, Radim Krčmář wrote:
>>> >                                                                (Hint: we
>>> > want kvm-unit-tests for this).
>> :) Something like this?
>> 
>>   enable_xapic()
>>   id = apic_id()
>>   set_apic_id(id+1) // ?
>>   enable_x2apic()
>>   id == apic_id() & 0xff
>>   disable_apic()
>>   enable_xapic()
>>   id == apic_id()
>> 
> 
> Yes, plus checking that it "moves" appropriately between low and high bits.

x2APIC cannot use MMIO interface, so apic_id() already does the best we
can ... if KVM is shifting wrong somwhere, then the id should differ.