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

[Shameerali Kolothum Thodi]

> -----Original Message-----
> From: Nathan Chen <nathanc@nvidia.com>
> Sent: Friday, November 22, 2024 1:42 AM
> 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;
> 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; Nicolin Chen <nicolinc@nvidia.com>
> Subject: Re: [RFC PATCH 0/5] hw/arm/virt: Add support for user-creatable
> nested SMMUv3
> 
>  >> Also as a heads up, I've added support for auto-inserting PCIe switch
>  >> between the PXB and GPUs in libvirt to attach multiple devices to a
> SMMU
>  >> node per libvirt's documentation - "If you intend to plug multiple
>  >> devices into a pcie-expander-bus, you must connect a
>  >> pcie-switch-upstream-port to the pcie-root-port that is plugged into the
>  >> pcie-expander-bus, and multiple pcie-switch-downstream-ports to the
>  >> pcie-switch-upstream-port". Future unit-tests should follow this
>  >> topology configuration.
>  >
>  > Ok. Could you please give me an example Qemu equivalent command
> option,
>  > if possible, for the above case. I am not that familiar with libvirt
> and I would
>  > also like to test the above scenario.
> 
> You can use "-device x3130-upstream" for the upstream switch port, and
> "-device xio3130-downstream" for the downstream port:
> 
>   -device pxb-pcie,bus_nr=250,id=pci.1,bus=pcie.0,addr=0x1 \
>   -device pcie-root-port,id=pci.2,bus=pci.1,addr=0x0 \
>   -device x3130-upstream,id=pci.3,bus=pci.2,addr=0x0 \
>   -device xio3130-
> downstream,id=pci.4,bus=pci.3,addr=0x0,chassis=17,port=1 \
>   -device vfio-pci,host=0009:01:00.0,id=hostdev0,bus=pci.4,addr=0x0 \
>   -device arm-smmuv3-nested,pci-bus=pci.1

Thanks. Just wondering why libvirt mandates usage of pcie-switch for multiple
device plugging rather than just using pcie-root-ports?

Please let me if there is any advantage in doing so that you are aware of.

Thanks,
Shameer