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

Jason Wang
2024-10-18T04:55:03Z

On Thu, Oct 17, 2024 at 4:53 PM Srujana Challa <schalla@marvell.com> wrote:
>
> > Subject: Re: [EXTERNAL] Re: [PATCH v2 0/2] vhost-vdpa: Add support for NO-
> > IOMMU mode
> >
> > On Wed, Oct 16, 2024 at 05:28:27PM +0000, Srujana Challa wrote:
> > > 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.
> >
> > While making the system completely unsafe to use. Maybe you should fix
> > your stack to use the iommu more itelligently instead?
>
> We observed better performance with "intel_iommu=on" in high-end x86 machines,
> indicating that the performance limitations are specific to low-end x86 hardware.

Do you have any analysis on why Intel IOMMU is slow on "low-end" x86
hardware? Anyhow, we can ask Intel IOMMU experts to help here.

Thanks

> This presents a trade-off between performance and security. Since intel_iommu
> is enabled by default, users who prioritize security over performance do not need to
> disable this option.