linux-kernel - Re: [PATCH v3] KVM: remove buggy vcpu id check on vcpu creation

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Message-ID: <20160422130705.GD7202@potion>
Date:	Fri, 22 Apr 2016 15:07:06 +0200
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Wanpeng Li <kernellwp@...il.com>
Cc:	Greg Kurz <gkurz@...ux.vnet.ibm.com>,
	Paolo Bonzini <pbonzini@...hat.com>, james.hogan@...tec.com,
	Ingo Molnar <mingo@...hat.com>, linux-mips@...ux-mips.org,
	kvm <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	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 v3] KVM: remove buggy vcpu id check on vcpu creation

2016-04-22 09:40+0800, Wanpeng Li:
> 2016-04-21 23:29 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
>> x86 vcpu_id encodes APIC ID and APIC ID encodes CPU topology by
>> reserving blocks of bits for socket/core/thread, so if core or thread
>> count isn't a power of two, then the set of valid APIC IDs is sparse,
> 
>              ^^^^^^^^^^^^^^^^^^^
>              ^^^^^^^
> Is this the root reason why recommand max vCPUs per vm is 160 and the
> KVM_MAX_VCPUS is 255 instead of due to perforamnce concern?

No, the recommended amout of VCPUs is 160 because I didn't bump it after
PLE stopped killing big guests. :/

You can get full 255 VCPU guest with a proper configuration, e.g.
"-smp 255" or "-smp 255,cores=8" and the only problem is scalability,
but I don't know of anything that doesn't scale to that point.

(Scaling up to 2^32 is harder, because you don't want O(N) search, nor
 full allocation on smaller guests.  Neither is a big problem now.)