Re: [EXTERNAL] Re: [PATCH v2 0/2] vhost-vdpa: Add support for NO-IOMMU mode

Michael S. Tsirkin
2024-10-16T17:41:59Z

On Mon, Oct 14, 2024 at 08:48:27PM -0700, Christoph Hellwig wrote:
> On Mon, Oct 14, 2024 at 01:18:01PM +0000, Srujana Challa wrote:
> > > On Fri, Sep 20, 2024 at 07:35:28PM +0530, Srujana Challa wrote:
> > > > This patchset introduces support for an UNSAFE, no-IOMMU mode in the
> > > > vhost-vdpa driver. When enabled, this mode provides no device
> > > > isolation, no DMA translation, no host kernel protection, and cannot
> > > > be used for device assignment to virtual machines. It requires RAWIO
> > > > permissions and will taint the kernel.
> > > >
> > > > This mode requires enabling the
> > > "enable_vhost_vdpa_unsafe_noiommu_mode"
> > > > option on the vhost-vdpa driver and also negotiate the feature flag
> > > > VHOST_BACKEND_F_NOIOMMU. This mode would be useful to get better
> > > > performance on specifice low end machines and can be leveraged by
> > > > embedded platforms where applications run in controlled environment.
> > >
> > > ... and is completely broken and dangerous.
> > Based on the discussions in this thread https://www.spinics.net/lists/kvm/msg357569.html,
> > we have decided to proceed with this implementation. Could you please share any
> > alternative ideas or suggestions you might have?
>
> Don't do this. It is inherently unsafe and dangerous and there is not
> valid reason to implement it.
>
> Double-Nacked-by: Christoph Hellwig <hch@lst.de>

It's basically because vfio does, so we have to follow suit.

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MST