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

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Message-ID: <CAHP4M8XQw5_2LX4OpYeO+8bbAEEaRmjQ39+nPzk0qXzwG7uaUQ@mail.gmail.com>
Date: Sat, 20 Sep 2025 08:34:41 +0530
From: Ajay Garg <ajaygargnsit@...il.com>
To: Alex Williamson <alex.williamson@...hat.com>
Cc: iommu@...ts.linux-foundation.org, linux-pci@...r.kernel.org, 
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: How are iommu-mappings set up in guest-OS for dma_alloc_coherent

> If the VM is configured without a vIOMMU or the vIOMMU is inactive in
> the guest, all of the guest physical memory is pinned and mapped
> through the physical IOMMU when the guest is started.  Nothing happens
> regarding the IOMMU when a coherent mapping is created in the guest,
> it's already setup.
>

Thanks Alex.

Another doubt pops up for this scenario.

Let's take a host-OS, with two guess-OSes spawned up (we can take
everything to be x86_64 for simplicity).
Guest G1 has PCI-device-1 attached to it; Guest G2 has PCI-device-2
attached to it.

a)
We do "dma_alloc_coherent" in G1, which returns GVA1 (CPU
virtual-address) and GIOVA1 (Device-bus virtual-address).
Since vIOMMU is not exposed in guest, so GIOVA1 will/can be equal to
GPA1 (physical-address).

This GIOVA1 (== GPA1) will be programmed to the PCI-device-1's
BAR-register to set up DMA.

b)
Similarly, we do "dma_alloc_coherent" in G2, and program GIOVA2 (==
GPA2) to PCI-device-2's BAR-register to set up DMA.

c)
At this point, the physical/host IOMMU will contain mappings for :

    GIOVA1 => HPA1
    GIOVA2 => HPA2

We take "sufficiently" multi-core systems, so that both guests could
be running concurrently, and HPA1 != HPA2 generally.
However, since both guests are running independently, we could very
well land in the situation where

    GIOVA1 == GIOVA2 (== GPA1 == GPA2).

How do we handle such conflicts?
Does x86-IOMMU-PASID come to the rescue here (implicitly meaning that
PCI-device-1 and PCI-device-2 """"must"""" be PASID capable)?

Once again, many thanks for your time and help Alex !

Thanks and Regards,
Ajay