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

Srujana Challa
2024-10-16T17:28:40Z

> Subject: Re: [EXTERNAL] Re: [PATCH v2 0/2] vhost-vdpa: Add support for NO-
> IOMMU mode
>
> 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
>
> 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://urldefense.proofpoint.com/v2/url?u=https-3A__www.spinics.net_l
> >
> ists_kvm_msg357569.html&d=DwIBAg&c=nKjWec2b6R0mOyPaz7xtfQ&r=Fj4O
> oD5hcK
> >
> FpANhTWdwQzjT1Jpf7veC5263T47JVpnc&m=Kj2YVdoGecovW95oPf_fILveQer
> 4EsJfWJ
> >
> XmzmACF_v1jROPwW343ZXF2nEc5JN7&s=Dw7EoKg_W8Ak7E0uGR4gT3vHBv
> Em2uEP1Pvx0
> > 5dXVHI&e=, 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>

When using the DPDK virtio user PMD, we’ve noticed a significant 70%
performance improvement when IOMMU is disabled on specific low-end
x86 machines. This performance improvement can be particularly
advantageous for embedded platforms where applications operate in
controlled environments. Therefore, we believe supporting the intel_iommu=off
mode is beneficial.

Thanks.