linux-kernel - Re: [PATCH 1/1] iommu/vt-d: Add Kconfig option to enable/disable scalable mode

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Date:   Thu, 7 Nov 2019 01:34:36 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     Joerg Roedel <joro@...tes.org>,
        David Woodhouse <dwmw2@...radead.org>, kevin.tian@...el.com,
        ashok.raj@...el.com, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org, jacob.jun.pan@...el.com
Subject: Re: [PATCH 1/1] iommu/vt-d: Add Kconfig option to enable/disable
 scalable mode

On Wed, Nov 06, 2019 at 01:11:30PM +0800, Lu Baolu wrote:
> This adds a Kconfig option INTEL_IOMMU_SCALABLE_MODE_ON to make
> it easier for distributions to enable or disable the Intel IOMMU
> scalable mode during kernel build.



> 
> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
> ---
>  drivers/iommu/Kconfig       | 10 ++++++++++
>  drivers/iommu/intel-iommu.c |  5 +++++
>  2 files changed, 15 insertions(+)
> 
> diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
> index e3842eabcfdd..32f30e27791c 100644
> --- a/drivers/iommu/Kconfig
> +++ b/drivers/iommu/Kconfig
> @@ -242,6 +242,16 @@ config INTEL_IOMMU_FLOPPY_WA
>  	  workaround will setup a 1:1 mapping for the first
>  	  16MiB to make floppy (an ISA device) work.
>  
> +config INTEL_IOMMU_SCALABLE_MODE_ON

That should have a DEFAULT in the name as it is a default.

> +	def_bool n

n is the default default, so this can just be bool.

> +#ifdef CONFIG_INTEL_IOMMU_SCALABLE_MODE_ON
> +int intel_iommu_sm = 1;
> +#else
>  int intel_iommu_sm;
> +#endif /* CONFIG_INTEL_IOMMU_SCALABLE_MODE_ON */

This can use IS_ENABLED().

But then again the distro can just add iommu=sm_on to CONFIG_CMDLINE
and have the same effect, so I don't really get the point of the whole
patch.

Or why we can't just enable it by default for that matter.