[PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array

From	Radim Krčmář <rkrcmar@redhat.com>
Newsgroups	linux.kernel
Subject	[PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array
Date	2017-08-16 21:50 +0200
Message-ID	<ufcud-2Ob-1@gated-at.bofh.it> (permalink)
Organization	linux.* mail to news gateway

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The goal is to increase KVM_MAX_VCPUS without worrying about memory
impact of many small guests.

This is a second out of three major "dynamic" options:
 1) size vcpu array at VM creation time
 2) resize vcpu array when new VCPUs are created
 3) use a lockless list/tree for VCPUs

The disadvantage of (1) is its requirement on userspace changes and
limited flexibility because userspace must provide the maximal count on
start.  The main advantage is that kvm->vcpus will work like it does
now.  It has been posted as "[PATCH 0/4] KVM: add KVM_CREATE_VM2 to
allow dynamic kvm->vcpus array",
http://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1377285.html

The main problem of (2), this series, is that we cannot extend the array
in place and therefore require some kind of protection when moving it.
RCU seems best, but it makes the code slower and harder to deal with.
The main advantage is that we do not need userspace changes.

The third option wasn't explored yet.  It would solve the ugly
kvm_for_each_vcpu() of (2), but kvm_get_vcpu() would become linear.
(We could mitigate it by having list of vcpu arrays and A lockless
 sequentially growing "tree" would be logarithmic and not that much more
 complicated to implement.)

Which option do you think is the best?

Thanks.

Radim Krčmář (2):
  KVM: remove unused __KVM_HAVE_ARCH_VM_ALLOC
  KVM: RCU protected dynamic vcpus array

 arch/mips/kvm/mips.c       |  8 +++--
 arch/powerpc/kvm/powerpc.c |  6 ++--
 arch/s390/kvm/kvm-s390.c   | 27 ++++++++++++-----
 arch/x86/kvm/hyperv.c      |  3 +-
 arch/x86/kvm/vmx.c         |  3 +-
 arch/x86/kvm/x86.c         |  5 ++--
 include/linux/kvm_host.h   | 73 +++++++++++++++++++++++++++++-----------------
 virt/kvm/arm/arm.c         | 10 +++----
 virt/kvm/kvm_main.c        | 72 +++++++++++++++++++++++++++++++++++++--------
 9 files changed, 144 insertions(+), 63 deletions(-)

-- 
2.13.3

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[PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array Radim Krčmář <rkrcmar@redhat.com> - 2017-08-16 21:50 +0200
  [PATCH RFC 1/2] KVM: remove unused __KVM_HAVE_ARCH_VM_ALLOC Radim Krčmář <rkrcmar@redhat.com> - 2017-08-16 21:50 +0200
    Re: [PATCH RFC 1/2] KVM: remove unused __KVM_HAVE_ARCH_VM_ALLOC Christian Borntraeger <borntraeger@de.ibm.com> - 2017-08-21 15:50 +0200
  [PATCH RFC 2/2] KVM: RCU protected dynamic vcpus array Radim Krčmář <rkrcmar@redhat.com> - 2017-08-16 21:50 +0200
    Re: [PATCH RFC 2/2] KVM: RCU protected dynamic vcpus array Cornelia Huck <cohuck@redhat.com> - 2017-08-17 10:10 +0200
    Re: [PATCH RFC 2/2] KVM: RCU protected dynamic vcpus array David Hildenbrand <david@redhat.com> - 2017-08-17 13:20 +0200
      Re: [PATCH RFC 2/2] KVM: RCU protected dynamic vcpus array Paolo Bonzini <pbonzini@redhat.com> - 2017-08-17 19:00 +0200
      Re: [PATCH RFC 2/2] KVM: RCU protected dynamic vcpus array Radim Krčmář <rkrcmar@redhat.com> - 2017-08-17 19:00 +0200
  Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array Alexander Graf <agraf@suse.de> - 2017-08-17 09:10 +0200
    Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array Cornelia Huck <cohuck@redhat.com> - 2017-08-17 09:40 +0200
      Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array Paolo Bonzini <pbonzini@redhat.com> - 2017-08-17 11:20 +0200
        Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array Cornelia Huck <cohuck@redhat.com> - 2017-08-17 11:30 +0200
          Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array Paolo Bonzini <pbonzini@redhat.com> - 2017-08-17 11:50 +0200
            Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array David Hildenbrand <david@redhat.com> - 2017-08-17 12:00 +0200
              Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array Paolo Bonzini <pbonzini@redhat.com> - 2017-08-17 12:20 +0200
                Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array David Hildenbrand <david@redhat.com> - 2017-08-17 12:30 +0200
                Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array Paolo Bonzini <pbonzini@redhat.com> - 2017-08-17 12:30 +0200
                Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array David Hildenbrand <david@redhat.com> - 2017-08-17 12:40 +0200
    Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array Radim Krčmář <rkrcmar@redhat.com> - 2017-08-17 17:00 +0200
      Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array Alexander Graf <agraf@suse.de> - 2017-08-17 21:20 +0200
        Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array Radim Krčmář <rkrcmar@redhat.com> - 2017-08-18 16:20 +0200
          Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array Marc Zyngier <marc.zyngier@arm.com> - 2017-08-18 16:30 +0200
  Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array Cornelia Huck <cohuck@redhat.com> - 2017-08-17 09:40 +0200
  Re: [PATCH RFC 0/2] KVM: use RCU to allow dynamic kvm->vcpus array David Hildenbrand <david@redhat.com> - 2017-08-17 09:40 +0200

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