Passthrough a HDD without iommu

[lxiosjao]

Hi,

I have a question about iommu.

When I do things like :

qm set 107 -scsi0 /dev/disk/by-id/ata-Western.......

Why is it possible to passthrough this sata hdd to the vm without enabling IOMMU ?
Whats the difference between passthrough a hdd and passthrough an entire controler ?
Am I supposed to enable IOMMU to passthrough a HDD to a VM ?
And add this in /etc/modules :

vfio
vfio_iommu_type1
vfio_pci
vfio_virqfd

Thanks for the help

 

[leesteken]

Why is it possible to passthrough this sata hdd to the vm without enabling IOMMU ?
Whats the difference between passthrough a hdd and passthrough an entire controler ?
Am I supposed to enable IOMMU to passthrough a HDD to a VM ?

It is possible because you are not passing through a PCI(e) device, such as a SATA controller. If you passthrough a controller, then all drives connected to it are also removed from the host because the controller is moved to the VM, and hopefully the VM will see those connected drives.
The difference is that you do not passthrough a device, it only provides the VM access to a block storage area, like a (whole) disk drive or just a partition of a drive. This is also the reason you won't see any SMART information in the VM or the serial number of the HDD and it will show as a QEMU HARDDISK.
PCI(e) passthrough and disk passthrough are entirely separate concepts (although they sound similar). You do not need to enable IOMMU to passthrough a single disk or partition. You do need IOMMU to passthrough entire PCI(e) devices, like GPUs and USB, network and SATA controllers

 

[lxiosjao]

Thanks for your answer.
So we can admit that it's not a "real passthrough" when the command : "qm set 107 -scsi0 /dev/disk/by-id/ata-Western" is used ?
- What about performance ? (pci passthrough vs provide a disk access to a vm)
- Can I backup the whole disk I provide to the vm from the GUI in the option ?

 

[leesteken]

What is "real passthrough"? There is PCI(e) passthrough, USB passthrough and block device passthrough (where you use /dev/disk/...).
Performance depends on a lot of things. Try various options and test what works best for your setup. I don't know about backups, sorry.

 

[Dunuin]

Thanks for your answer.
So we can admit that it's not a "real passthrough" when the command : "qm set 107 -scsi0 /dev/disk/by-id/ata-Western" is used ?

That is basically like using a virtual disk, so it is still virtualized by KVM/virtio, but instead of storing the virtual disk on a LV/Zvol it will store the blockdata directly on that physical disk/partition.
So using PCI passthrough should be faster because the VM can directly and physically access the drives without any virtualization in between so there should be no overhead at all. But atleast with virtio SCSI the overhead should be quite low so "qm set" passthrough should be fine in most cases.