linux-kernel - Re: [RFC PATCH 1/3] content: Add VIRTIO_F_SWIOTLB to negotiate use of SWIOTLB bounce buffers

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Message-ID: <20250403040643-mutt-send-email-mst@kernel.org>
Date: Thu, 3 Apr 2025 04:13:59 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: David Woodhouse <dwmw2@...radead.org>
Cc: Zhu Lingshan <lingshan.zhu@....com>, virtio-comment@...ts.linux.dev,
	hch@...radead.org, Claire Chang <tientzu@...omium.org>,
	linux-devicetree <devicetree@...r.kernel.org>,
	Rob Herring <robh+dt@...nel.org>,
	Jörg Roedel <joro@...tes.org>,
	iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	graf@...zon.de
Subject: Re: [RFC PATCH 1/3] content: Add VIRTIO_F_SWIOTLB to negotiate use
 of SWIOTLB bounce buffers

On Thu, Apr 03, 2025 at 08:54:45AM +0100, David Woodhouse wrote:
> On Thu, 2025-04-03 at 03:34 -0400, Michael S. Tsirkin wrote:
> > 
> > Indeed I personally do not exactly get why implement a virtual system
> > without an IOMMU when virtio-iommu is available.
> > 
> > I have a feeling it's about lack of windows drivers for virtio-iommu
> > at this point.
> 
> And a pKVM (etc.) implementation of virtio-iommu which would allow the
> *trusted* part of the hypervisor to know which guest memory should be
> shared with the VMM implementing the virtio device models?

Is there a blocker here?

> You'd also end up in a situation where you have a virtio-iommu for some
> devices, and a real two-stage IOMMU (e.g. SMMU or AMD's vIOMMU) for
> other devices. Are guest operating systems going to cope well with
> that?

They should. In particular because systems with multiple IOMMUs already
exist.

> Do the available discovery mechanisms for all the relevant IOMMUs
> even *allow* for that to be expressed?

I think yes. But, it's been a while since I played with this, let me
check what works, what does not, and get back to you on this.

-- 
MST