linux-kernel - Re: [PATCH v1 03/11] KVM: x86: dynamic kvm_apic

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Date:	Fri, 1 Jul 2016 16:03:02 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Radim Krčmář <rkrcmar@...hat.com>
Cc:	Andrew Honig <ahonig@...gle.com>, linux-kernel@...r.kernel.org,
	kvm <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 03/11] KVM: x86: dynamic kvm_apic_map



On 01/07/2016 14:44, Radim Krčmář wrote:
> 2016-07-01 10:42+0200, Paolo Bonzini:
>> On 01/07/2016 00:15, Andrew Honig wrote:
>>>>> +       /* kvm_apic_map_get_logical_dest() expects multiples of 16 */
>>>>> +       size = round_up(max_id + 1, 16);
>>> Now that you're using the full range of apic_id values, could this
>>> calculation overflow?  Perhaps max_id could be u64?
>>
>> Good point, but I wonder if it's a good idea to let userspace allocate
>> 32 GB of memory. :)
> 
> Yes, both could happen.  I'll change it to u64 to make it future proof.

It's not necessary to change it to u64 if you put a limit, but you can
add a WARN_ON(size == 0).

Also if kvm_apic_map_get_logical_dest() expects multiples of 16, it
should warn whenever the invariant is not respected.

>> Let's put a limit on the maximum supported APIC ID, and report it
>> through KVM_CHECK_EXTENSION on the new KVM_CAP_X2APIC_API capability.
>> If 767 is enough for Knights Landing, the allocation below fits in two
>> pages.  If you need to make it higher, please change the allocation to
>> use kvm_kvzalloc and kvfree.
> 
> We sort of have a capability for maximum APIC ID, KVM_MAX_VCPU_ID,
> because VCPU ID is initial APIC ID and x2APIC ID should always be the
> initial APIC ID.

Should it?  According to QEMU if you have e.g. 3 cores per socket one
socket take 4 APIC IDs.  For Knights Landing the "worst" prime factor in
288 is 3^2 so you need APIC IDs up to 288 * (4/3)^2 = 512.

Paolo