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

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Message-ID: <CAHP4M8X89SUY=qoSO3xy8-TE0ubWGkOqP-WwP6niyn+NQLKUvQ@mail.gmail.com>
Date: Mon, 22 Sep 2025 21:51:09 +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

>
> VFIO doesn't make PCI devices disappear from the host.  Maybe you're
> referring to unbinding the host function driver, which might make your
> NIC/HBA/GPU device disappear from the host as the PCI device is bound
> to vfio-pci instead.
>

Yep Alex, that's what I meant.
I am sorry for (unintentionally) causing ambiguity.

> There are ways to multiplex devices between host and guest, SR-IOV is
> currently the most common way to do this.  Here you'd have a physical
> function (PF) with a host function driver, which can create multiple
> virtual functions (VFs), each of which have a unique requester ID and
> therefore a unique set of page tables allowing them to operate in
> independent IOVA spaces for VMs.  You can imagine here that your PF
> remains bound to the host function driver and continues to provide host
> services, while the VFs can be assigned to VMs.

Perfect, thanks Alex ..

>
> PASID is another way to do this and is often described in an SIOV
> (Scalable IOV) framework, where we rely more on software to expose an
> assignable entity which makes use of the combination of the physical
> requester ID along with PASID to create a unique IOVA space through two
> levels of IOMMU page tables.

Perfect again, many thanks again Alex ..

>
> In either case, having an SR-IOV or PASID capability on the device
> doesn't automatically enable device multiplexing, there's software
> required to enable these features, more so in the direction of SIOV
> support as the scalability trade-off is to push more of the basic
> device emulation into software.

Thanks a ton Alex for all the help !
Thank you for always being there whenever we get stuck .. !!


Thanks and Regards,
Ajay