Re: [PATCH] iommu/amd: Add kernel parameters to limit V1 page-sizes

[Baolu Lu]

On 2024/9/5 15:22, Joerg Roedel wrote:
> From: Joerg Roedel<jroedel@xxxxxxx>
>
> Add two new kernel command line parameters to limit the page-sizes
> used for v1 page-tables:
>
> 	nohugepages     - Limits page-sizes to 4KiB
>
> 	v2_pgsizes_only - Limits page-sizes to 4Kib/2Mib/1GiB; The
> 	                  same as the sizes used with v2 page-tables
>
> This is needed for multiple scenarios. When assigning devices to
> SEV-SNP guests the IOMMU page-sizes need to match the sizes in the RMP
> table, otherwise the device will not be able to access all shared
> memory.
>
> Also, some ATS devices do not work properly with arbitrary IO
> page-sizes as supported by AMD-Vi, so limiting the sizes used by the
> driver is a suitable workaround.
>
> All-in-all, these parameters are only workarounds until the IOMMU core
> and related APIs gather the ability to negotiate the page-sizes in a
> better way.
>
> Signed-off-by: Joerg Roedel<jroedel@xxxxxxx>
> ---
>   Documentation/admin-guide/kernel-parameters.txt | 17 +++++++++++------
>   drivers/iommu/amd/amd_iommu.h                   |  1 +
>   drivers/iommu/amd/amd_iommu_types.h             |  4 ++++
>   drivers/iommu/amd/init.c                        |  8 ++++++++
>   drivers/iommu/amd/io_pgtable.c                  |  2 +-
>   5 files changed, 25 insertions(+), 7 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 09126bb8cc9f..6d6630aec46c 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -333,12 +333,17 @@
>   					  allowed anymore to lift isolation
>   					  requirements as needed. This option
>   					  does not override iommu=pt
> -			force_enable - Force enable the IOMMU on platforms known
> -				       to be buggy with IOMMU enabled. Use this
> -				       option with care.
> -			pgtbl_v1     - Use v1 page table for DMA-API (Default).
> -			pgtbl_v2     - Use v2 page table for DMA-API.
> -			irtcachedis  - Disable Interrupt Remapping Table (IRT) caching.
> +			force_enable    - Force enable the IOMMU on platforms known
> +				          to be buggy with IOMMU enabled. Use this
> +				          option with care.
> +			pgtbl_v1        - Use v1 page table for DMA-API (Default).
> +			pgtbl_v2        - Use v2 page table for DMA-API.
> +			irtcachedis     - Disable Interrupt Remapping Table (IRT) caching.
> +			nohugepages     - Limit page-sizes used for v1 page-tables
> +				          to 4 KiB.

Intel iommu driver has a similar option 'intel_iommu=sp_off'

	sp_off [Default Off]
		By default, super page will be supported if Intel IOMMU
		has the capability. With this option, super page will
		not be supported.

Is it possible to consolidate these two into a single
"iommu.nohugepages=1"?

> +			v2_pgsizes_only - Limit page-sizes used for v1 page-tables
> +				          to 4KiB/2Mib/1GiB.
> +
>   
>   	amd_iommu_dump=	[HW,X86-64]
>   			Enable AMD IOMMU driver option to dump the ACPI table

Thanks,
baolu