Re: [PATCH V2 01/10] accel/kvm: Extract common KVM vCPU {creation,parking} code

[Jonathan Cameron]

On Sat, 30 Sep 2023 01:19:24 +0100
Salil Mehta <salil.mehta@huawei.com> wrote:

> 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>

Hi Salil,

A few trivial things inline, plus a question about why 
cpu->cpu_index can now be used but kvm_arch_vcpu_id(cpu);
was previously needed.

Thanks,

Jonathan

> ---
>  accel/kvm/kvm-all.c  | 63 +++++++++++++++++++++++++++++++++-----------
>  include/sysemu/kvm.h | 14 ++++++++++
>  2 files changed, 61 insertions(+), 16 deletions(-)
> 
> diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
> index ff1578bb32..b8c36ba50a 100644
> --- a/accel/kvm/kvm-all.c
> +++ b/accel/kvm/kvm-all.c
> @@ -80,7 +80,7 @@
>  #endif
>  
>  struct KVMParkedVcpu {
> -    unsigned long vcpu_id;
> +    int vcpu_id;
>      int kvm_fd;
>      QLIST_ENTRY(KVMParkedVcpu) node;
>  };
> @@ -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, int vcpu_id);
>  
>  static inline void kvm_resample_fd_remove(int gsi)
>  {
> @@ -320,11 +321,49 @@ err:
>      return ret;
>  }
>  
> +void kvm_park_vcpu(CPUState *cpu)
> +{
> +    int vcpu_id = cpu->cpu_index;
> +    struct KVMParkedVcpu *vcpu;
> +
> +    vcpu = g_malloc0(sizeof(*vcpu));
> +    vcpu->vcpu_id = vcpu_id;

As vcpu_id is only used here why have the local variable?
Maybe that changes in later patches, in which case ignore this.

    vcpu->vcpu_id = cpu->cpu_index;

Why is kvm_arch_vcpu_id() not necessary here any more but was
before?

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

See below. I'm not sure why it's safe not to use kvm_arch_vcpu_id()
Seems a few architectures have less than trivial implementations of
that function currently.


> +    KVMState *s = kvm_state;
> +    int kvm_fd;
> +
> +    DPRINTF("kvm_create_vcpu\n");
> +
> +    /* check if the KVM vCPU already exist but is parked */
> +    kvm_fd = kvm_get_vcpu(s, vcpu_id);
> +    if (kvm_fd < 0) {
> +        /* vCPU not parked: create a new KVM vCPU */
> +        kvm_fd = kvm_vm_ioctl(s, KVM_CREATE_VCPU, vcpu_id);
> +        if (kvm_fd < 0) {
> +            error_report("KVM_CREATE_VCPU IOCTL failed for vCPU %d", 
> vcpu_id);
> +            return kvm_fd;
> +        }
> +    }
> +
> +    cpu->vcpu_dirty = true;
> +    cpu->kvm_fd = kvm_fd;
> +    cpu->kvm_state = s;
> +    cpu->dirty_pages = 0;
> +    cpu->throttle_us_per_full = 0;

Trivial but I would have maintained the order wrt to the code removed
below just to avoid a reviewer having to check the two bits of code
do the same thing after the reorder.

> +
> +    return 0;
> +}
> +
>  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 +392,7 @@ 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;
>  }
> @@ -369,7 +405,7 @@ void kvm_destroy_vcpu(CPUState *cpu)
>      }
>  }
>  
> -static int kvm_get_vcpu(KVMState *s, unsigned long vcpu_id)
> +static int kvm_get_vcpu(KVMState *s, int vcpu_id)
>  {
>      struct KVMParkedVcpu *cpu;
>  
> @@ -384,7 +420,7 @@ 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 +431,14 @@ int kvm_init_vcpu(CPUState *cpu, Error **errp)
>  
>      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);

The switch from kvm_arch_vcpu_id(cpu) to using 
int vcpu_id = cpu->cpu_index;

Seems like a functional change on some arch.
>      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)",

The rewrap of the lines above seems like an unrelated change.

>                           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..785f3ed083 100644
> --- a/include/sysemu/kvm.h
> +++ b/include/sysemu/kvm.h
> @@ -464,6 +464,20 @@ 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);
> +/**
> + * kvm_create_vcpu - Gets a parked KVM vCPU or creates a KVM vCPU
> + * @cpu:  QOM CPUState object for which KVM vCPU has to be created/fetched.

Extra space before QOM (same below)

> + *
> + * @returns: 0 when success, errno (<0) when failed.
> + */
> +int kvm_create_vcpu(CPUState *cpu);

Blank line here perhaps.

> +/**
> + * kvm_park_vcpu - Gets a parked KVM vCPU if it exists
> + * @cpu:  QOM CPUState object for which parked KVM vCPU has to be fetched.

We aren't returning anything, so why fetch?

> + *
> + * @returns: kvm_fd (>0) when success, -1 when failed.
> + */
> +void kvm_park_vcpu(CPUState *cpu);
>  
>  #endif /* NEED_CPU_H */
>