Re: [PATCH 05/37] KVM: Record the executing ioctl number on the vcpu struct

[Andrew Jones <drjones@xxxxxxxxxx>]

On Thu, Oct 12, 2017 at 12:41:09PM +0200, Christoffer Dall wrote:
> Some architectures may decide to do different things during
> kvm_arch_vcpu_load depending on the ioctl being executed.  For example,
> arm64 is about to do significant work in vcpu load/put when running a
> vcpu, but not when doing things like KVM_SET_ONE_REG or
> KVM_SET_MP_STATE.
> 
> Therefore, store the ioctl number that we are executing on the VCPU
> during the first vcpu_load() which succeeds in getting the vcpu->mutex
> and set the ioctl number to 0 when exiting kvm_vcpu_ioctl() after
> successfully loading the vcpu.
> 
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Radim Krčmář <rkrcmar@xxxxxxxxxx>
> Signed-off-by: Christoffer Dall <christoffer.dall@xxxxxxxxxx>
> ---
>  arch/x86/kvm/vmx.c       | 2 +-
>  arch/x86/kvm/x86.c       | 8 ++++----
>  include/linux/kvm_host.h | 3 ++-
>  virt/kvm/kvm_main.c      | 6 ++++--
>  4 files changed, 11 insertions(+), 8 deletions(-)
>

I wonder if enough other architectures would be able to benefit from this
for most/all of their non-RUN VCPU ioctls. If so, then maybe we should
consider doing something like

 int __vcpu_load(struct kvm_vcpu *vcpu)
 {
    int cpu;

    cpu = get_cpu();
    preempt_notifier_register(&vcpu->preempt_notifier);
    kvm_arch_vcpu_load(vcpu, cpu);
    put_cpu();
    return 0;
 }
 int vcpu_load(struct kvm_vcpu *vcpu)
 {
    if (mutex_lock_killable(&vcpu->mutex))
        return -EINTR;
    return __vcpu_load(vcpu);
 }

and the equivalent for vcpu_put.

Then just take the lock in kvm_vcpu_ioctl and leave it to the
kvm_arch_vcpu_ioctl_* functions to call __vcpu_load/__vcpu_put
if necessary.

Thanks,
drew