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

[Shameerali Kolothum Thodi]

Hi Daniel,

> -----Original Message-----
> From: Daniel P. Berrangé <berrange@redhat.com>
> Sent: Thursday, January 30, 2025 4:00 PM
> To: Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com>
> Cc: qemu-arm@nongnu.org; qemu-devel@nongnu.org;
> eric.auger@redhat.com; peter.maydell@linaro.org; jgg@nvidia.com;
> 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
> Subject: Re: [RFC PATCH 0/5] hw/arm/virt: Add support for user-creatable
> nested SMMUv3
> 
> On Fri, Nov 08, 2024 at 12:52:37PM +0000, Shameer Kolothum via wrote:
> > How to use it(Eg:):
> >
> > On a HiSilicon platform that has multiple physical SMMUv3s, the ACC ZIP
> VF
> > devices and HNS VF devices are behind different SMMUv3s. So for a
> Guest,
> > specify two smmuv3-nested devices each behind a pxb-pcie as below,
> >
> > ./qemu-system-aarch64 -machine virt,gic-version=3,default-bus-bypass-
> iommu=on \
> > -enable-kvm -cpu host -m 4G -smp cpus=8,maxcpus=8 \
> > -object iommufd,id=iommufd0 \
> > -bios QEMU_EFI.fd \
> > -kernel Image \
> > -device virtio-blk-device,drive=fs \
> > -drive if=none,file=rootfs.qcow2,id=fs \
> > -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-nested,id=smmuv1,pci-bus=pcie.1 \
> > -device vfio-pci,host=0000:7d:02.1,bus=pcie.port1,iommufd=iommufd0 \
> > -device pxb-pcie,id=pcie.2,bus_nr=16,bus=pcie.0 \
> > -device pcie-root-port,id=pcie.port2,bus=pcie.2,chassis=2 \
> > -device arm-smmuv3-nested,id=smmuv2,pci-bus=pcie.2 \
> > -device vfio-pci,host=0000:75:00.1,bus=pcie.port2,iommufd=iommufd0 \
> > -append "rdinit=init console=ttyAMA0 root=/dev/vda2 rw
> earlycon=pl011,0x9000000" \
> > -device virtio-9p-pci,fsdev=p9fs2,mount_tag=p9,bus=pcie.0 \
> > -fsdev local,id=p9fs2,path=p9root,security_model=mapped \
> > -net none \
> > -nographic
> 
> Above you say the host has 2 SMMUv3 devices, and you've created 2
> SMMUv3
> guest devices to match.
> 
> The various emails in this thread & libvirt thread, indicate that each
> guest SMMUv3 is associated with a host SMMUv3, but I don't see any
> property on the command line for 'arm-ssmv3-nested' that tells it which
> host eSMMUv3 it is to be associated with.
> 
> How does this association work ?

You are right. The association is not very obvious in Qemu. The association
and checking is done implicitly by kernel at the moment.  I will try to explain
it here.

Each "arm-smmuv3-nested" instance, when the first device gets attached
to it, will create a S2 HWPT and a corresponding SMMUv3 domain in kernel
SMMUv3 driver. This domain will have a pointer representing the physical
SMMUv3 that the device belongs. And any other device which belongs to
the same physical SMMUv3 can share this S2 domain.

If a device that belongs to a different physical SMMUv3 gets attached to
the above domain, the HWPT attach will eventually fail as the physical
smmuv3 in the domains will have a mismatch,
https://elixir.bootlin.com/linux/v6.13/source/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c#L2860

And as I mentioned in cover letter, Qemu will report,

"
Attempt to add the HNS VF to a different SMMUv3 will result in,

-device vfio-pci,host=0000:7d:02.2,bus=pcie.port3,iommufd=iommufd0: Unable to 
attach viommu
-device vfio-pci,host=0000:7d:02.2,bus=pcie.port3,iommufd=iommufd0: vfio 
0000:7d:02.2:
   Failed to set iommu_device: [iommufd=29] error attach 0000:7d:02.2 (38) to 
id=11: Invalid argument

At present Qemu is not doing any extra validation other than the above 
failure to make sure the user configuration is correct or not. The
assumption is libvirt will take care of this.
"
So in summary, if the libvirt gets it wrong, Qemu will fail with error.

If a more explicit association is required, some help from kernel is required
to identify the physical SMMUv3 associated with the device.  

Jason/Nicolin, any thoughts on this?

Thanks,
Shameer