Re: [RFC PATCH 0/5] hw/arm/virt: Add support for user-creatable nested SMMUv3

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

On 2/6/25 9:53 AM, Daniel P. Berrangé wrote:
> On Fri, Jan 31, 2025 at 05:08:28PM +0100, Eric Auger wrote:
>> Hi,
>>
>>
>> On 1/31/25 4:23 PM, Shameerali Kolothum Thodi wrote:
>>>> -----Original Message-----
>>>> From: Jason Gunthorpe <jgg@nvidia.com>
>>>> Sent: Friday, January 31, 2025 2:54 PM
>>>> To: Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com>
>>>> Cc: Daniel P. Berrangé <berrange@redhat.com>; qemu-arm@nongnu.org;
>>>> qemu-devel@nongnu.org; eric.auger@redhat.com;
>>>> peter.maydell@linaro.org; nicolinc@nvidia.com; ddutile@redhat.com;
>>>> Linuxarm <linuxarm@huawei.com>; Wangzhou (B)
>>>> <wangzhou1@hisilicon.com>; jiangkunkun <jiangkunkun@huawei.com>;
>>>> Jonathan Cameron <jonathan.cameron@huawei.com>;
>>>> zhangfei.gao@linaro.org; Nathan Chen <nathanc@nvidia.com>
>>>> Subject: Re: [RFC PATCH 0/5] hw/arm/virt: Add support for user-creatable
>>>> nested SMMUv3
>>>>
>>>> On Fri, Jan 31, 2025 at 02:39:53PM +0000, Shameerali Kolothum Thodi
>>>> wrote:
>>>>
>>>>>>> And Qemu does some checking to make sure that the device is indeed
>>>>>> associated
>>>>>>> with the specified phys-smmuv3.  This can be done going through the
>>>>>> sysfs path checking
>>>>>>> which is what I guess libvirt is currently doing to populate the
>>>> topology.
>>>>>> So basically
>>>>>>> Qemu is just replicating that to validate again.
>>>>>> I would prefer that iommufd users not have to go out to sysfs..
>>>>>>
>>>>>>> Or another option is extending the IOMMU_GET_HW_INFO IOCTL to
>>>>>> return the phys
>>>>>>> smmuv3 base address which can avoid going through the sysfs.
>>>>>> It also doesn't seem great to expose a physical address. But we could
>>>>>> have an 'iommu instance id' that was a unique small integer?
>>>>> Ok. But how the user space can map that to the device?
>>>> Why does it need to?
>>>>
>>>> libvirt picks some label for the vsmmu instance, it doesn't matter
>>>> what the string is.
>>>>
>>>> qemu validates that all of the vsmmu instances are only linked to PCI
>>>> device that have the same iommu ID. This is already happening in the
>>>> kernel, it will fail attaches to mismatched instances.
>>>>
>>>> Nothing further is needed?
>>> -device pxb-pcie,id=pcie.1,bus_nr=8,bus=pcie.0 \
>>> -device pcie-root-port,id=pcie.port1,bus=pcie.1,chassis=1 \
>>> -device arm-smmuv3-accel,bus=pcie.1,id=smmuv1 \
>> I don't get what is the point of adding such an id if it is not
>> referenced anywhere?
> Every QDev device instance has an 'id' property - if you don't
> set one explicitly, QEMU will generate one internally. Libvirt
> will always set the 'id' property to avoid the internal auto-
> generated IDs, as it wants full knowledge of naming.

OK thank you for the explanation

Eric
>
> With regards,
> Daniel