[Qemu-devel] intel-iommu and vhost: Do we need 'device-iotlb' and 'ats'?

[Jintack Lim]

Hi,

I'm using vhost with the virtual intel-iommu, and this page[1] shows
the QEMU command line example.

qemu-system-x86_64 -M q35,accel=kvm,kernel-irqchip=split -m 2G \
                   -device intel-iommu,intremap=on,device-iotlb=on \
                   -device ioh3420,id=pcie.1,chassis=1 \
                   -device
virtio-net-pci,bus=pcie.1,netdev=net0,disable-legacy=on,disable-modern=off,iommu_platform=on,ats=on
\
                   -netdev tap,id=net0,vhostforce \
                   $IMAGE_PATH

I wonder what's the impact of using device-iotlb and ats options as
they are described necessary.

In my understanding, vhost in the kernel only looks at
VIRTIO_F_IOMMU_PLATFORM, and when it is set, vhost uses a
device-iotlb. In addition, vhost and QEMU communicate using vhost_msg
basically to cache mappings correctly in the vhost, so I wonder what's
the role of ats in this case.

A related question is that if we use SMMU emulation[2] on ARM without
those options, does vhost cache mappings as if it has a device-iotlb?
(I guess this is the case.)

I'm pretty new to QEMU code, so I might be missing something. Can
somebody shed some light on it?

[1] https://wiki.qemu.org/Features/VT-d
[2] http://lists.nongnu.org/archive/html/qemu-devel/2018-02/msg04736.html

Thanks,
Jintack