linux-kernel - Re: [RFC v1 0/5] Hyper-V: Add para-virtualized IOMMU support for Linux guests

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Message-ID: <engj6x3koovijgbh6vvmki3xbd4dtetejenlcgu4bqg7bwi762@h2ovu62se4ee>
Date: Sat, 10 Jan 2026 13:39:18 +0800
From: Yu Zhang <zhangyu1@...ux.microsoft.com>
To: Michael Kelley <mhklinux@...look.com>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	"linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>, "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>, 
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>, "kys@...rosoft.com" <kys@...rosoft.com>, 
	"haiyangz@...rosoft.com" <haiyangz@...rosoft.com>, "wei.liu@...nel.org" <wei.liu@...nel.org>, 
	"decui@...rosoft.com" <decui@...rosoft.com>, "lpieralisi@...nel.org" <lpieralisi@...nel.org>, 
	"kwilczynski@...nel.org" <kwilczynski@...nel.org>, "mani@...nel.org" <mani@...nel.org>, 
	"robh@...nel.org" <robh@...nel.org>, "bhelgaas@...gle.com" <bhelgaas@...gle.com>, 
	"arnd@...db.de" <arnd@...db.de>, "joro@...tes.org" <joro@...tes.org>, 
	"will@...nel.org" <will@...nel.org>, "robin.murphy@....com" <robin.murphy@....com>, 
	"easwar.hariharan@...ux.microsoft.com" <easwar.hariharan@...ux.microsoft.com>, "jacob.pan@...ux.microsoft.com" <jacob.pan@...ux.microsoft.com>, 
	"nunodasneves@...ux.microsoft.com" <nunodasneves@...ux.microsoft.com>, "mrathor@...ux.microsoft.com" <mrathor@...ux.microsoft.com>, 
	"peterz@...radead.org" <peterz@...radead.org>, "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: [RFC v1 0/5] Hyper-V: Add para-virtualized IOMMU support for
 Linux guests

On Thu, Jan 08, 2026 at 06:45:52PM +0000, Michael Kelley wrote:
> From: Yu Zhang <zhangyu1@...ux.microsoft.com> Sent: Monday, December 8, 2025 9:11 PM
> > 
> > This patch series introduces a para-virtualized IOMMU driver for
> > Linux guests running on Microsoft Hyper-V. The primary objective
> > is to enable hardware-assisted DMA isolation and scalable device
> 
> Is there any particular meaning for the qualifier "scalable" vs. just
> "device assignment"? I just want to understand what you are getting
> at.
> 

Sorry for the ambiguity.
I intended to highlight two primary use cases for pvIOMMU:
- to enable in-kernel DMA protection within the guest.
- to allow device assignment to guest user space (e.g., via VFIO).

I avoided using the phrase "device assignment" alone, because people may be
confused if the main purpose of introducing pvIOMMU is for device assignment
to a L1 guest(which actually does not depend on any virtual IOMMU) or to a
L2 nested guest(altough I guess w/ pvIOMMU, it should work but we've never
tested that case and are not aware any such requirement).

And you are right, simply adding "scalable" didn't help clarify this.
I will rephrase the commit message. Thanks!

B.R.
Yu