Re: [PATCH 4/5] hw/arm/smmuv3: Move reset to exit phase

[Eric Auger <eric.auger@redhat.com>]

On 2/7/25 5:37 PM, Peter Maydell wrote:
> On Thu, 6 Feb 2025 at 14:23, Eric Auger <eric.auger@redhat.com> wrote:
>> Currently the iommu may be reset before the devices
>> it protects. For example this happens with virtio-scsi-pci.
>> when system_reset is issued from qmp monitor, spurious
>> "virtio: zero sized buffers are not allowed" warnings can
>> be observed.
>>
>> Signed-off-by: Eric Auger <eric.auger@redhat.com>
>> ---
>>  hw/arm/smmuv3.c     | 9 +++++----
>>  hw/arm/trace-events | 1 +
>>  2 files changed, 6 insertions(+), 4 deletions(-)
>>
>> diff --git a/hw/arm/smmuv3.c b/hw/arm/smmuv3.c
>> index c0cf5df0f6..7522c32b24 100644
>> --- a/hw/arm/smmuv3.c
>> +++ b/hw/arm/smmuv3.c
>> @@ -1870,13 +1870,14 @@ static void smmu_init_irq(SMMUv3State *s, SysBusDevice *dev)
>>      }
>>  }
>>
>> -static void smmu_reset_hold(Object *obj, ResetType type)
>> +static void smmu_reset_exit(Object *obj, ResetType type)
>>  {
>>      SMMUv3State *s = ARM_SMMUV3(obj);
>>      SMMUv3Class *c = ARM_SMMUV3_GET_CLASS(s);
>>
>> -    if (c->parent_phases.hold) {
>> -        c->parent_phases.hold(obj, type);
>> +    trace_smmu_reset_exit();
>> +    if (c->parent_phases.exit) {
>> +        c->parent_phases.exit(obj, type);
>>      }
> If we need to do something unexpected like reset
> register values in the exit phase rather than the
> hold phase, it's a good idea to have a comment explaining
> why, to avoid somebody coming along afterwards and tidying
> it up into the more usual arrangement.
sure
>
> If I understand correctly we need to keep the whole IOMMU
> config intact until the exit phase? What's the thing the
> device behind the IOMMU is trying to do during its reset
> that triggers the warning?
The virtio-pci-net continues to perform DMA requests and this causes
some weird messages such as:
"virtio: bogus descriptor or out of resources"

Also VFIO devices may continue issuing DMAs causing translation faults

Thanks

Eric

>
> thanks
> -- PMM
>