Re: [PATCH 1/1] arch/x86: Add CONFIG_IOMMU_DEFAULT_PT for iommu=pt

[Konrad Rzeszutek Wilk <konrad.wilk-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>]

On Thu, Sep 29, 2016 at 09:58:55AM -0700, Glenn Serre wrote:
> Add CONFIG_IOMMU_DEFAULT_PT for iommu=pt
> 
> Allow default to iommu=pt without adding to command line.
> Add iommu=nopt to turn off default.

What's the reason / use-case for it?

> 
> Signed-off-by: Glenn Serre <gserre-43mecJUBy8ZBDgjK7y7TUQ@public.gmane.org>
> ---
>  Documentation/kernel-parameters.txt | 3 ++-
>  arch/x86/Kconfig                    | 7 +++++++
>  arch/x86/kernel/pci-dma.c           | 6 ++++++
>  3 files changed, 15 insertions(+), 1 deletion(-)
> 
> --- a/Documentation/kernel-parameters.txt
> +++ b/Documentation/kernel-parameters.txt
> @@ -1607,7 +1607,8 @@
>  		nomerge
>  		forcesac
>  		soft
> -		pt		[x86, IA-64]
> +		pt		[x86]
> +		nopt		[x86]
>  		nobypass	[PPC/POWERNV]
>  			Disable IOMMU bypass, using IOMMU for PCI devices.
> 
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -845,6 +845,15 @@
>  	def_bool y
>  	depends on CALGARY_IOMMU || GART_IOMMU || SWIOTLB || AMD_IOMMU
> 
> +config IOMMU_DEFAULT_PT
> +	int "Default iommu pt value (0-1)"
> +	range 0 1
> +	default "0"
> +	prompt "Set to 1 to default to iommu=pt"
> +	help
> +	  Setting this option to 1 will set iommu passthrough (iommu=pt)
> +	  by default.  Use iommu=nopt to override.
> +
>  config MAXSMP
>  	bool "Enable Maximum number of SMP Processors and NUMA Nodes"
>  	depends on X86_64 && SMP && DEBUG_KERNEL
> --- a/arch/x86/kernel/pci-dma.c
> +++ b/arch/x86/kernel/pci-dma.c
> @@ -43,7 +43,7 @@
>   * useful if a user wants to use an IOMMU only for KVM device assignment to
>   * guests and not for driver dma translation.
>   */
> -int iommu_pass_through __read_mostly;
> +int iommu_pass_through __read_mostly = CONFIG_IOMMU_DEFAULT_PT;
> 
>  extern struct iommu_table_entry __iommu_table[], __iommu_table_end[];
> 
> @@ -196,6 +196,8 @@
>  #endif
>  		if (!strncmp(p, "pt", 2))
>  			iommu_pass_through = 1;
> +		if (!strncmp(p, "nopt", 4))
> +			iommu_pass_through = 0;
> 
>  		gart_parse_options(p);
> 
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