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

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Date:	Fri, 1 Jul 2016 10:42:00 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Andrew Honig <ahonig@...gle.com>,
	Radim Krčmář <rkrcmar@...hat.com>
Cc:	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 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. :)

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.

Last but not least...

>> > +       new = kzalloc(sizeof(struct kvm_apic_map) +
>> > +                     sizeof(struct kvm_lapic) * size, GFP_KERNEL);
                           ^^^^^^^^^^^^^^^^^^^^^^^^
... the sizeof here must be sizeof(struct kvm_lapic *).

Thanks,

Paolo