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    Subject[RFC PATCH 0/2] KVM: Support Heterogeneous RT VCPU Configurations.
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    Hello,

    This series attempts to solve some issues that arise from
    having some VCPUs be real-time while others aren't.

    We are trying to play media inside a VM on a desktop environment
    (Chromebooks), which requires us to have some tasks in the guest
    be serviced at real-time priority on the host so that the media
    can be played smoothly.

    To achieve this, we give a VCPU real-time priority on the host
    and use isolcpus= to ensure that only designated tasks are allowed
    to run on the RT VCPU.
    In order to avoid priority inversions (for example when the RT
    VCPU preempts a non-RT that's holding a lock that it wants to
    acquire), we dedicate a host core to the RT vcpu: Only the RT
    VCPU is allowed to run on that CPU, while all the other non-RT
    cores run on all the other host CPUs.

    This approach works on machines that have a large enough number
    of CPUs where it's possible to dedicate a whole CPU for this,
    but we also have machines that only have 2 CPUs and doing this
    on those is too costly.

    This patch series makes it possible to have a RT VCPU without
    having to dedicate a whole host core for it.
    It does this by making it so that non-RT VCPUs can't be
    preempted if they are in a critical section, which we
    approximate as having interrupts disabled or non-zero
    preempt_count. Once the VCPU is found to not be in a critical
    section anymore, it will give up the CPU.
    There measures to ensure that preemption isn't delayed too
    many times.

    (I realize that the hooks in the scheduler aren't very
    tasteful, but I couldn't figure out a better way.
    SVM support will be added when sending the patch for
    inclusion.)

    Feedback or alternatives are appreciated.

    Thanks,
    -- Suleiman


    Suleiman Souhlal (2):
    kvm,x86: Support heterogeneous RT VCPU configurations.
    kvm,x86: Report preempt_count to host.

    arch/x86/Kconfig | 11 +++++
    arch/x86/include/asm/kvm_host.h | 7 +++
    arch/x86/include/uapi/asm/kvm_para.h | 2 +
    arch/x86/kernel/kvm.c | 10 ++++
    arch/x86/kvm/Kconfig | 13 ++++++
    arch/x86/kvm/cpuid.c | 3 ++
    arch/x86/kvm/vmx/vmx.c | 15 ++++++
    arch/x86/kvm/x86.c | 70 +++++++++++++++++++++++++++-
    arch/x86/kvm/x86.h | 2 +
    include/linux/kvm_host.h | 4 ++
    include/linux/preempt.h | 7 +++
    kernel/sched/core.c | 30 ++++++++++++
    virt/kvm/Kconfig | 3 ++
    virt/kvm/kvm_main.c | 13 ++++++
    14 files changed, 189 insertions(+), 1 deletion(-)

    --
    2.32.0.432.gabb21c7263-goog

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