Re: [PATCH RFC 3/4] KVM: Define KVM_USER_MEM_SLOTS in arch-neutral include/linux/kvm_host.h

Sean Christopherson
2021-01-15T17:06:02Z

On Fri, Jan 15, 2021, Vitaly Kuznetsov wrote:
> Memory slots are allocated dynamically when added so the only real
> limitation in KVM is 'id_to_index' array which is 'short'. Define
> KVM_USER_MEM_SLOTS to the maximum possible value in the arch-neutral
> include/linux/kvm_host.h, architectures can still overtide the setting
> if needed.

Leaving the max number of slots nearly unbounded is probably a bad idea. If my
math is not completely wrong, this would let userspace allocate 6mb of kernel
memory per VM. Actually, worst case scenario would be 12mb since modifying
memslots temporarily has two allocations.

If we remove the arbitrary limit, maybe replace it with a module param with a
sane default?

> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
> include/linux/kvm_host.h | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index f3b1013fb22c..ab83a20a52ca 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -425,6 +425,10 @@ struct kvm_irq_routing_table {
> #define KVM_PRIVATE_MEM_SLOTS 0
> #endif
>
> +#ifndef KVM_USER_MEM_SLOTS
> +#define KVM_USER_MEM_SLOTS (SHRT_MAX - KVM_PRIVATE_MEM_SLOTS)
> +#endif
> +
> #ifndef KVM_MEM_SLOTS_NUM
> #define KVM_MEM_SLOTS_NUM (KVM_USER_MEM_SLOTS + KVM_PRIVATE_MEM_SLOTS)
> #endif
> --
> 2.29.2
>