Re: [PATCH v7 16/24] KVM: arm64: vgic-its: KVM_DEV_ARM_ITS_SAVE/RESTORE_TABLES

Auger Eric
2017-05-07T17:30:26Z

Hi,

On 07/05/2017 15:51, Marc Zyngier wrote:
> On Sun, May 07 2017 at 2:00:30 pm BST, Marc Zyngier <marc.zyngier@arm.com> wrote:
>> On Sat, May 06 2017 at 4:24:35 pm BST, Eric Auger <eric.auger@redhat.com> wrote:
>>> Introduce new attributes in KVM_DEV_ARM_VGIC_GRP_CTRL group:
>>> - KVM_DEV_ARM_ITS_SAVE_TABLES: saves the ITS tables into guest RAM
>>> - KVM_DEV_ARM_ITS_RESTORE_TABLES: restores them into VGIC internal
>>> structures.
>>>
>>> We hold the vcpus lock during the save and restore to make
>>> sure no vcpu is running.
>>>
>>> At this stage the functionality is not yet implemented. Only
>>> the skeleton is put in place.
>>>
>>> Signed-off-by: Eric Auger <eric.auger@redhat.com>
>>>
>>> ---
>>> v6 -> v7:
>>> - also hold the its_lock on save and restore
>>>
>>> v5 -> v6:
>>> - remove the pending table sync from the ITS table restore
>>>
>>> v4 -> v5:
>>> - use KVM_DEV_ARM_ITS_SAVE_TABLES and KVM_DEV_ARM_ITS_RESTORE_TABLES
>>> - rename *flush* into *save*
>>> - call its_sync_lpi_pending_table at the end of restore
>>> - use abi framework
>>>
>>> v3 -> v4:
>>> - pass kvm struct handle to vgic_its_flush/restore_pending_tables
>>> - take the kvm lock and vcpu locks
>>> - ABI revision check
>>> - check attr->attr is null
>>>
>>> v1 -> v2:
>>> - remove useless kvm parameter
>>> ---
>>> arch/arm/include/uapi/asm/kvm.h | 4 +-
>>> arch/arm64/include/uapi/asm/kvm.h | 4 +-
>>> virt/kvm/arm/vgic/vgic-its.c | 107 ++++++++++++++++++++++++++++++++++++--
>>> 3 files changed, 109 insertions(+), 6 deletions(-)
>>>
>>
>> [...]
>>
>>> @@ -1718,7 +1777,37 @@ static int vgic_its_save_tables_v0(struct vgic_its *its)
>>> */
>>> static int vgic_its_restore_tables_v0(struct vgic_its *its)
>>> {
>>> - return -ENXIO;
>>> + struct kvm *kvm = its->dev->kvm;
>>> + int ret;
>>> +
>>> + mutex_lock(&kvm->lock);
>>> + mutex_lock(&its->its_lock);
>>> +
>>> + if (!lock_all_vcpus(kvm)) {
>>> + mutex_unlock(&its->its_lock);
>>> + mutex_unlock(&kvm->lock);
>>> + return -EBUSY;
>>> + }
>>> +
>>> + ret = vgic_its_restore_collection_table(its);
>>> + if (ret)
>>> + goto out;
>>> +
>>> + ret = vgic_its_restore_device_tables(its);
>>> +
>>> +out:
>>> + unlock_all_vcpus(kvm);
>>> + mutex_unlock(&its->its_lock);
>>> + mutex_unlock(&kvm->lock);
>>> +
>>> + if (ret)
>>> + return ret;
>>> +
>>> + /*
>>> + * On restore path, MSI injections can happen before the
>>> + * first VCPU run so let's complete the GIC init here.
>>> + */
>>> + return kvm_vgic_map_resources(its->dev->kvm);
>>
>> This one is still a rather sore spot, but I've lost track of what we can
>> do about it. Until now, this would only happen on first run. But it
>> looks like QEMU and KVM have different views of what "runable" is.
>>
>> I'm not sure there is a good way to solve this, unfortunately. From a
>> device PoV, everything is ready and the fact that nothing is clocking
>> the CPUs is very much irrelevant. I'm almost tempted to say that the
>> map_resource() call in kvm_vcpu_first_run_init shouldn't be there, and
>> that we're missing a synchronization point with userspace that would say
>> "system is entierely configured", triggering the iodev registration.
>>
>> Oh well. At that point, and short of having something better to offer:
>>
>> Acked-by: Marc Zyngier <marc.zyngier@arm.com>
>>
>> M.
>
> Actually, there is a rather big problem here. This function is called on
> a per-ITS basis. But once we have run map_resources *once*, any other
> call becomes ineffective (vgic_ready() returns true). What happens to
> the other ITSs? Can they still be successfully restored? Don't they
> end-up with the same "blind spot" you've tried to close?

The 1st call to map_resources() maps GICV3 regs and also maps all ITSes.
So I don't think the problem comes from the fact subsequent calls are
inefficient (and return 0) but that for this 1st call to succeeds all
ITS base addresses must have been provided by the userspace. Otherwise
this restoration of this ITS will fail. So this is not explicitly
documented that way in the ABI doc.

Thanks

Eric
>
> Thanks,
>
> M.
>