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

[David Woodhouse <dwmw2@infradead.org>]

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On Thu, 2025-04-03 at 04:13 -0400, Michael S. Tsirkin wrote:
> 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?

Only the amount of complexity in what should be a minimal Trusted
Compute Base. (And ideally subject to formal methods of proving its
correctness too.)

And frankly, if we were going to accept a virtio-iommu in the TCB why
not just implement enough virtqueue knowledge to build something where
the trusted part just snoops on the *actual* e.g. virtio-net device to
know which buffers the VMM was *invited* to access, and facilitate
that?

We looked at doing that. It's awful.

> > 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.

Even if it could work in theory, I'll be astonished if it actually
works in practice across a wide set of operating systems, and if it
*ever* works for Windows.

Compared with the simple option of presenting a device which
conceptually doesn't even *do* DMA, which is confined to its own
modular device driver... 

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