linux-kernel - How are iommu-mappings set up in guest-OS for dma_alloc

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Message-ID: <CAHP4M8W+uMHkzcx-fHJ0NxYf4hrkdFBQTGWwax5wHLa0Qf37Nw@mail.gmail.com>
Date: Thu, 18 Sep 2025 23:24:19 +0530
From: Ajay Garg <ajaygargnsit@...il.com>
To: iommu@...ts.linux-foundation.org, linux-pci@...r.kernel.org, 
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: How are iommu-mappings set up in guest-OS for dma_alloc_coherent

Hi everyone.

Let's say we have a following setup :

i)
x86_64 host-os, booted up with iommu enabled and pass-through mode.

ii)
x86_64 guest-os, booted up using vfio+qemu+kvm and a pci-device attached to it.

iii)
A guest-os-device-driver calls "dma_alloc_coherent", after which the
returned dma-address / iova is programmed to the pci-device's
mmio-register.

In the above case, how are the IOMMU mappings set up during the
guest-os-device-driver's "dma_alloc_coherent" call?
Does :

a)
The VMM / KVM intercept the "dma_alloc_coherent" call, and use the
host-iommu to set up things?

OR

b)
There is no interception from VMM / KVM, but rather the guest-OS
itself has a view of the IOMMU (through the regular ACPI tables
populated during guest boot up)?

OR

c)
Anything else under the hood?

Will be grateful for clearing the haze.

Thanks and Regards,
Ajay