Re: [PATCH 1/9] accel/kvm: Extract common KVM vCPU {creation,parking} code

["Alex Bennée" <alex.bennee@linaro.org>]

Salil Mehta <salil.mehta@huawei.com> writes:

> KVM vCPU creation is done once during the initialization of the VM when Qemu
> threads are spawned. This is common to all the architectures.
>
> Hot-unplug of vCPU results in destruction of the vCPU objects in QOM but
> the KVM vCPU objects in the Host KVM are not destroyed and their representative
> KVM vCPU objects/context in Qemu are parked.
>
> Refactor common logic so that some APIs could be reused by vCPU Hotplug code.
>
> Signed-off-by: Salil Mehta <salil.mehta@huawei.com>
> ---
>  accel/kvm/kvm-all.c  | 61 ++++++++++++++++++++++++++++++++++----------
>  include/sysemu/kvm.h |  2 ++
>  2 files changed, 49 insertions(+), 14 deletions(-)
>
> diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
> index ff1578bb32..57889c5f6c 100644
> --- a/accel/kvm/kvm-all.c
> +++ b/accel/kvm/kvm-all.c
> @@ -137,6 +137,7 @@ static QemuMutex kml_slots_lock;
>  #define kvm_slots_unlock()  qemu_mutex_unlock(&kml_slots_lock)
>  
>  static void kvm_slot_init_dirty_bitmap(KVMSlot *mem);
> +static int kvm_get_vcpu(KVMState *s, unsigned long vcpu_id);

weird to have a forward static declaration here if you are declare in
the header later on.

>  
>  static inline void kvm_resample_fd_remove(int gsi)
>  {
> @@ -320,11 +321,51 @@ err:
>      return ret;
>  }
>  
> +void kvm_park_vcpu(CPUState *cpu)
> +{
> +    unsigned long vcpu_id = cpu->cpu_index;

cpu_index is a plain int in CPUState:

    int cpu_index;

but is defined as an unsigned long in KVMParkedVcpu:

    unsigned long vcpu_id;

I'm not sure if this is just a historical anomaly but I suspect we
should fixup the discrepancy first so all users of cpu_index use the
same size.

> +    struct KVMParkedVcpu *vcpu;
> +
> +    vcpu = g_malloc0(sizeof(*vcpu));
> +    vcpu->vcpu_id = vcpu_id;
> +    vcpu->kvm_fd = cpu->kvm_fd;
> +    QLIST_INSERT_HEAD(&kvm_state->kvm_parked_vcpus, vcpu, node);
> +}
> +
> +int kvm_create_vcpu(CPUState *cpu)
> +{

I don't think you get anything other than -1 on failure so at this point
you might as well return a bool.

> +    unsigned long vcpu_id = cpu->cpu_index;

Is this used?

> +    KVMState *s = kvm_state;
> +    int ret;
> +
> +    DPRINTF("kvm_create_vcpu\n");

Please don't add new DPRINTFs - use tracepoints instead. Whether its
worth cleaning up up kvm-all first I leave up to you. 

> +    /* check if the KVM vCPU already exist but is parked */
> +    ret = kvm_get_vcpu(s, kvm_arch_vcpu_id(cpu));
> +    if (ret > 0) {
> +        goto found;
> +    }
> +
> +    /* create a new KVM vCPU */
> +    ret = kvm_vm_ioctl(s, KVM_CREATE_VCPU, (void *)vcpu_id);
> +    if (ret < 0) {
> +        return ret;
> +    }
> +
> +found:
> +    cpu->vcpu_dirty = true;
> +    cpu->kvm_fd = ret;
> +    cpu->kvm_state = s;
> +    cpu->dirty_pages = 0;
> +    cpu->throttle_us_per_full = 0;
> +
> +    return 0;
> +}
> +

This is trivially nestable to avoid gotos:

  bool kvm_create_vcpu(CPUState *cpu)
  {
      unsigned long vcpu_id = cpu->cpu_index;
      KVMState *s = kvm_state;
      int ret;

      /* check if the KVM vCPU already exist but is parked */
      ret = kvm_get_vcpu(s, kvm_arch_vcpu_id(cpu));
      if (ret < 0) {
          /* not found, try to create a new KVM vCPU */
          ret = kvm_vm_ioctl(s, KVM_CREATE_VCPU, (void *)vcpu_id);
          if (ret < 0) {
              return false;
          }
      }

      cpu->vcpu_dirty = true;
      cpu->kvm_fd = ret;
      cpu->kvm_state = s;
      cpu->dirty_pages = 0;
      cpu->throttle_us_per_full = 0;

      return true;
  }

>  static int do_kvm_destroy_vcpu(CPUState *cpu)
>  {
>      KVMState *s = kvm_state;
>      long mmap_size;
> -    struct KVMParkedVcpu *vcpu = NULL;
>      int ret = 0;
>  
>      DPRINTF("kvm_destroy_vcpu\n");
> @@ -353,10 +394,7 @@ static int do_kvm_destroy_vcpu(CPUState *cpu)
>          }
>      }
>  
> -    vcpu = g_malloc0(sizeof(*vcpu));
> -    vcpu->vcpu_id = kvm_arch_vcpu_id(cpu);
> -    vcpu->kvm_fd = cpu->kvm_fd;
> -    QLIST_INSERT_HEAD(&kvm_state->kvm_parked_vcpus, vcpu, node);
> +    kvm_park_vcpu(cpu);
>  err:
>      return ret;
>  }
> @@ -384,7 +422,7 @@ static int kvm_get_vcpu(KVMState *s, unsigned long vcpu_id)
>          }
>      }
>  
> -    return kvm_vm_ioctl(s, KVM_CREATE_VCPU, (void *)vcpu_id);
> +    return -1;
>  }
>  
>  int kvm_init_vcpu(CPUState *cpu, Error **errp)
> @@ -395,19 +433,14 @@ int kvm_init_vcpu(CPUState *cpu, Error **errp)

Hmm it looks like no one cares about the return value here and the fact
that callers use &error_fatal should be enough to exit. You can then
just return early and avoid the error ladder.

>  
>      trace_kvm_init_vcpu(cpu->cpu_index, kvm_arch_vcpu_id(cpu));
>  
> -    ret = kvm_get_vcpu(s, kvm_arch_vcpu_id(cpu));
> +    ret = kvm_create_vcpu(cpu);
>      if (ret < 0) {
> -        error_setg_errno(errp, -ret, "kvm_init_vcpu: kvm_get_vcpu failed (%lu)",
> +        error_setg_errno(errp, -ret,
> +                         "kvm_init_vcpu: kvm_create_vcpu failed (%lu)",
>                           kvm_arch_vcpu_id(cpu));
>          goto err;
>      }
>  
> -    cpu->kvm_fd = ret;
> -    cpu->kvm_state = s;
> -    cpu->vcpu_dirty = true;
> -    cpu->dirty_pages = 0;
> -    cpu->throttle_us_per_full = 0;
> -
>      mmap_size = kvm_ioctl(s, KVM_GET_VCPU_MMAP_SIZE, 0);
>      if (mmap_size < 0) {
>          ret = mmap_size;
> diff --git a/include/sysemu/kvm.h b/include/sysemu/kvm.h
> index ee9025f8e9..17919567a8 100644
> --- a/include/sysemu/kvm.h
> +++ b/include/sysemu/kvm.h
> @@ -464,6 +464,8 @@ void kvm_set_sigmask_len(KVMState *s, unsigned int sigmask_len);
>  
>  int kvm_physical_memory_addr_from_host(KVMState *s, void *ram_addr,
>                                         hwaddr *phys_addr);
> +int kvm_create_vcpu(CPUState *cpu);
> +void kvm_park_vcpu(CPUState *cpu);

Please add kdoc comments for the public API functions describing their
usage and parameters.

>  
>  #endif /* NEED_CPU_H */


-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro