Re: Make intel iommu disable swiotlb

[Baolu Lu <baolu.lu@linux.intel.com>]

On 2025/1/26 13:09, Adrian Mardare (amardare) wrote:
> (resending as plain-text)
> Hi...
> I'd like to submit the below patch.
> While investigating a difference in memory consumption between the kernel in 5.4.x and in 6.6.x,
> I noticed there is around ~80M less reported in MemTotal in 6.6.x versus 5.4.x.
> This is because  in 6.6.x  the swiotlb is still enabled and consuming about 80M of kernel memory, even though
> the intel iommu has been fully initialized.
> This change will cause the swiotlb to be disabled /  memory to be released ( via swiotlb_exit ).
> Thanks,
> Adrian
> 
> 
>  From 27e63e727f23189e9c98d81fb445141f998c0808 Mon Sep 17 00:00:00 2001
> From: Adrian Mardare<amardare@cisco.com>
> Date: Sat, 25 Jan 2025 23:41:31 -0500
> Subject: [PATCH] Make intel iommu disable swiotlb
> 
> On boards where intel iommu has been successfully
> initialized, we need to disable the swiotlb as there
> is no reason for swiotlb to be active past that.
> 
> Signed-off-by: Adrian Mardare<amardare@cisco.com>
> ---
> drivers/iommu/intel/iommu.c | 3 +++
> 1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> index cc46098f875b..971023d33a70 100644
> --- a/drivers/iommu/intel/iommu.c
> +++ b/drivers/iommu/intel/iommu.c
> @@ -3166,6 +3166,9 @@ int __init intel_iommu_init(void)
>                 pr_info("Intel(R) Virtualization Technology for Directed I/O\n");
> 
>                  intel_iommu_enabled = 1;
> +#ifdef CONFIG_SWIOTLB
> +             x86_swiotlb_enable = false;
> +#endif

Why does an iommu driver need to care about swiotlb? The extra memory
consumption might be a problem, but I don't think it should be fixed in
the iommu driver.

Thanks,
baolu