Re: [PATCH v8 05/17] KVM: kvm_io_bus: add kvm_io_bus_get_dev() call

André Przywara
2016-07-06T21:36:36Z

Hi Christoffer,

On 06/07/16 22:15, Christoffer Dall wrote:
> On Tue, Jul 05, 2016 at 12:22:57PM +0100, Andre Przywara wrote:
>> The kvm_io_bus framework is a nice place of holding information about
>> various MMIO regions for kernel emulated devices.
>> Add a call to retrieve the kvm_io_device structure which is associated
>> with a certain MMIO address. This avoids to duplicate kvm_io_bus'
>> knowledge of MMIO regions without having to fake MMIO calls if a user
>> needs the device a certain MMIO address belongs to.
>> This will be used by the ITS emulation to get the associated ITS device
>> when someone triggers an MSI via an ioctl from userspace.
>>
>> Signed-off-by: Andre Przywara <andre.przywara@arm.com>
>> Reviewed-by: Eric Auger <eric.auger@redhat.com>
>> ---
>> include/linux/kvm_host.h | 2 ++
>> virt/kvm/kvm_main.c | 24 ++++++++++++++++++++++++
>> 2 files changed, 26 insertions(+)
>>
>> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
>> index 0640ee9..614a981 100644
>> --- a/include/linux/kvm_host.h
>> +++ b/include/linux/kvm_host.h
>> @@ -164,6 +164,8 @@ int kvm_io_bus_register_dev(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
>> int len, struct kvm_io_device *dev);
>> int kvm_io_bus_unregister_dev(struct kvm *kvm, enum kvm_bus bus_idx,
>> struct kvm_io_device *dev);
>> +struct kvm_io_device *kvm_io_bus_get_dev(struct kvm *kvm, enum kvm_bus bus_idx,
>> + gpa_t addr);
>>
>> #ifdef CONFIG_KVM_ASYNC_PF
>> struct kvm_async_pf {
>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>> index ef54b4c..bd2eb92 100644
>> --- a/virt/kvm/kvm_main.c
>> +++ b/virt/kvm/kvm_main.c
>> @@ -3496,6 +3496,30 @@ int kvm_io_bus_unregister_dev(struct kvm *kvm, enum kvm_bus bus_idx,
>> return r;
>> }
>>
>
> do you need to hold kvm->slots_lock here like the other functions
> touching the io_bus framework here, which have comments specifying that?

AFAICT this comment is outdated, the slots_lock needs only to be taken
if one changes the kvm_io_bus (adding or removing devices).
Readers (looking up a device) use RCU. I looked at the other readers,
also in other architectures, none of them takes the lock AFAICS when
they just get an entry.

Paolo, Radim, can you confirm this?
Shall I send a patch that remove the misleading comments
at virt/kvm/kvm_main.c, lines 3347, 3365 and 3414?

Cheers,
Andre.

>> +struct kvm_io_device *kvm_io_bus_get_dev(struct kvm *kvm, enum kvm_bus bus_idx,
>> + gpa_t addr)
>> +{
>> + struct kvm_io_bus *bus;
>> + int dev_idx, srcu_idx;
>> + struct kvm_io_device *iodev = NULL;
>> +
>> + srcu_idx = srcu_read_lock(&kvm->srcu);
>> +
>> + bus = srcu_dereference(kvm->buses[bus_idx], &kvm->srcu);
>> +
>> + dev_idx = kvm_io_bus_get_first_dev(bus, addr, 1);
>> + if (dev_idx < 0)
>> + goto out_unlock;
>> +
>> + iodev = bus->range[dev_idx].dev;
>> +
>> +out_unlock:
>> + srcu_read_unlock(&kvm->srcu, srcu_idx);
>> +
>> + return iodev;
>> +}
>> +EXPORT_SYMBOL_GPL(kvm_io_bus_get_dev);
>> +
>> static struct notifier_block kvm_cpu_notifier = {
>> .notifier_call = kvm_cpu_hotplug,
>> };
>> --
>> 2.9.0
>>
>
> You also need an ack here, but my comment-comment above notwithstanding:
>
> Acked-by: Christoffer Dall <christoffer.dall@linaro.org>
>
> -Christoffer
>