Re: [PATCH RFC V2 05/37] accel/kvm: Extract common KVM vCPU {creation,parking} code

[Gavin Shan]

Hi Salil,

On 9/26/23 20:04, Salil Mehta 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. If the 
> architecture
> supports vCPU hot-{un}plug then this KVM vCPU creation could be deferred to
> later point as well. Some architectures might in any case create KVM vCPUs for
> the yet-to-be plugged vCPUs (i.e. QoM Object & thread does not exists) during VM
> init time and park them.
>
> 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 in Qemu are parked.
>
> 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(-)

The most important point seems missed in the commit log: The KVM vCPU objects,
including those hotpluggable objects, need to be in place before in-host GICv3
is initialized. So we need expose kvm_create_vcpu() to make those KVM vCPU
objects in place, even for those non-present vCPUs.

> diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
> index 7b3da8dc3a..86e9c9ea60 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);
>   
>   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;
> +    struct KVMParkedVcpu *vcpu;
> +
> +    vcpu = g_malloc0(sizeof(*vcpu));
> +    vcpu->vcpu_id = vcpu_id;

       vcpu->vcpu_id = cpu->cpu_index;

@vcpu_id can be dropped.

> +    vcpu->kvm_fd = cpu->kvm_fd;
> +    QLIST_INSERT_HEAD(&kvm_state->kvm_parked_vcpus, vcpu, node);
> +}
> +
> +int kvm_create_vcpu(CPUState *cpu)
> +{
> +    unsigned long vcpu_id = cpu->cpu_index;
> +    KVMState *s = kvm_state;
> +    int ret;
> +
> +    DPRINTF("kvm_create_vcpu\n");
> +
> +    /* 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;
> +}
> +

The found tag can be dropped. @cpu can be initialized if vCPU fd is found
and then bail early.

       /* The KVM vCPU may have been existing */
       ret = kvm_get_vcpu(s, kvm_arch_vcpu_id(cpu));
       if (ret > 0) {
           cpu->vcpu_dirty = true;
            :
            :
           return 0;
       }

       /* Create a new KVM vCPU */

>   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)
>   
>       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 115f0cca79..2c34889b01 100644
> --- a/include/sysemu/kvm.h
> +++ b/include/sysemu/kvm.h
> @@ -473,6 +473,8 @@ 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);
>   
>   #endif /* NEED_CPU_H */
>   

Thanks,
Gavin