RE: [RFC PATCH v2 00/20] hw/arm/virt: Add support for user-creatable accelerated SMMUv3

[Shameerali Kolothum Thodi via <qemu-devel@nongnu.org>]

Hi Eric,

> -----Original Message-----
> From: Eric Auger <eric.auger@redhat.com>
> Sent: Tuesday, March 25, 2025 2:43 PM
> To: Shameerali Kolothum Thodi
> <shameerali.kolothum.thodi@huawei.com>; qemu-arm@nongnu.org;
> qemu-devel@nongnu.org
> Cc: peter.maydell@linaro.org; jgg@nvidia.com; nicolinc@nvidia.com;
> ddutile@redhat.com; berrange@redhat.com; nathanc@nvidia.com;
> mochs@nvidia.com; smostafa@google.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 v2 00/20] hw/arm/virt: Add support for user-
> creatable accelerated SMMUv3
> 


> For the record I tested the series with host VFIO device and a
> virtio-blk-pci device put behind the same pxb-pcie/smmu protection and
> it works just fine
> 
> -+-[0000:0a]-+-01.0-[0b]----00.0  Mellanox Technologies ConnectX Family
> mlx5Gen Virtual Function
>  |           \-01.1-[0c]----00.0  Red Hat, Inc. Virtio 1.0 block device
>  \-[0000:00]-+-00.0  Red Hat, Inc. QEMU PCIe Host bridge
>              +-01.0-[01]--
>              +-01.1-[02]--
>              \-02.0  Red Hat, Inc. QEMU PCIe Expander bridge
> 
> This shows that without vcmdq feature there is no blocker having the
> same smmu device protecting both accelerated and emulated devices.

Thanks for giving it a spin. Yes, it currently supports the above. 

At the moment we are not using the IOTLB for the emulated dev for a
config like above.  Have you checked performance for either emulated or
vfio dev with the above config? Whatever light tests I have done it shows
performance degradation for emulated dev compared to the default
SMMUv3(iommu=smmuv3). 

And if the emulated dev issues _TLBI_NH_ASID, the code currently will propagate
that down to host SMMUv3. This will affect the vfio dev as well.

So the question is whether we want to allow this(assuming user is educated) or
block such a config as user has an option of using a non-accel smmuv3 for
emulated devices.

Thanks,
Shameer