linux-kernel - Re: [PATCH v4 2/2] KVM: move vcpu id checking to archs

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Message-ID: <20160422134837.GA30844@potion>
Date:	Fri, 22 Apr 2016 15:48:38 +0200
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Igor Mammedov <imammedo@...hat.com>
Cc:	Greg Kurz <gkurz@...ux.vnet.ibm.com>,
	Paolo Bonzini <pbonzini@...hat.com>, james.hogan@...tec.com,
	mingo@...hat.com, linux-mips@...ux-mips.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	David Hildenbrand <dahi@...ux.vnet.ibm.com>,
	qemu-ppc@...gnu.org, Cornelia Huck <cornelia.huck@...ibm.com>,
	Paul Mackerras <paulus@...ba.org>,
	David Gibson <david@...son.dropbear.id.au>
Subject: Re: [PATCH v4 2/2] KVM: move vcpu id checking to archs

2016-04-22 13:19+0200, Igor Mammedov:
> On Fri, 22 Apr 2016 11:25:38 +0200
> Greg Kurz <gkurz@...ux.vnet.ibm.com> wrote:
>> On Thu, 21 Apr 2016 19:36:11 +0200
>> Radim Krčmář <rkrcmar@...hat.com> wrote:
>> > 2016-04-21 18:45+0200, Greg Kurz:  
>> > > On Thu, 21 Apr 2016 18:00:19 +0200
>> > > Radim Krčmář <rkrcmar@...hat.com> wrote:    
>> > >> 2016-04-21 16:20+0200, Greg Kurz:    
>[...]
>> > >                                                                      maybe later
>> > > if we have other scenarios where vcpu ids need to cross the limit ?    
>> > 
>> > x86 is going to have that soon too -- vcpu_id will be able to range from
>> > 0 to 2^32-1 (or 2^31), but MAX_CPUS related data structures probably
>> > won't be improved to actually scale, so MAX_CPUS will remain lower.
>> >  
> That's not true, x86 is going to stick with KVM_MAX_VCPUS/qemu's max_cpus,
> the only thing that is going to change is that max supported APIC ID
> value will be in range 0 to 2^32-1 vs current 8bit one
> and since APIC ID is not vcpu_id so it won't affect vcpu_id.

I wish it wasn't.  vcpu_id is the initial APIC ID -- at least the spec
says so and KVM code behaves like that (QEMU does too).

It doesn't have to be so, though, KVM_SET_LAPIC provides the interface
to set APIC ID.  We'd decouple these two and change some related things.
(And add yet another cap for that? :])

I'll see what would be really needed,

thanks.