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

[Salil Mehta]

Hi Alex,
Thanks for taking pains to review it.

> From: Alex Bennée <alex.bennee@linaro.org>
> Sent: Friday, September 29, 2023 2:32 PM
> To: Salil Mehta <salil.mehta@huawei.com>
> Cc: qemu-devel@nongnu.org; qemu-arm@nongnu.org; maz@kernel.org; jean-
> philippe@linaro.org; Jonathan Cameron <jonathan.cameron@huawei.com>;
> lpieralisi@kernel.org; peter.maydell@linaro.org;
> richard.henderson@linaro.org; imammedo@redhat.com; andrew.jones@linux.dev;
> david@redhat.com; philmd@linaro.org; eric.auger@redhat.com;
> oliver.upton@linux.dev; pbonzini@redhat.com; mst@redhat.com;
> will@kernel.org; gshan@redhat.com; rafael@kernel.org;
> linux@armlinux.org.uk; darren@os.amperecomputing.com;
> ilkka@os.amperecomputing.com; vishnu@os.amperecomputing.com;
> karl.heubaum@oracle.com; miguel.luis@oracle.com; salil.mehta@opnsrc.net;
> zhukeqian <zhukeqian1@huawei.com>; wangxiongfeng (C)
> <wangxiongfeng2@huawei.com>; wangyanan (Y) <wangyanan55@huawei.com>;
> jiakernel2@gmail.com; maobibo@loongson.cn; lixianglai@loongson.cn; Linuxarm
> <linuxarm@huawei.com>
> Subject: Re: [PATCH 1/9] accel/kvm: Extract common KVM vCPU
> {creation,parking} code
> 
> 
> 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.

Yes, it is awkward. Actually, I had to move a function to get
a diff of changes which were more readable. Without moving
existing kvm_get_vcpu() function, change was not getting
highlighted properly in the generated patch. This relocation
of the function meant the order of declaration and its call
got reversed. So I had to use above tactics, even though weird.

Suggestions welcome to solve this?

> >  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.


Yes, agreed. But it is out of scope of this patch.


> > +    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.

kvm_vm_ioctl() can return other values? The 'ret' value can then be set 
using error_setg_errno() for example, in kvm_init_vcpu().

It is better to keep error handling standard for any future changes
as well.


> 
> > +    unsigned long vcpu_id = cpu->cpu_index;
> 
> Is this used?

It is used but we can get away with this. I think it
is a stray left over from shuffle.

Thanks!

> > +    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.

I can definitely change for this code.


> 
> > +    /* 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:


No issues. I can remove gotos. I was trying to reduce
indentation.


> 
>   bool kvm_create_vcpu(CPUState *cpu)

It is better to return ioctl value so that error can be
set at the caller handling code.

>   {
>       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;
>           }
>       }

Yes, can be done this way.

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

return value is better than Boolean.

>   }
> 
> >  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.

Maybe yes, but is that change really required?

> 
> >
> >      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.

Sure.


Thanks
Salil.