* Re: [PATCH v2 RESEND] x86/pci-dma: swiotlb: Fix a regression since 5.19.0 about iommu=soft kernel parameter
2026-10-04 10:12 ` [PATCH v2 RESEND] x86/pci-dma: swiotlb: Fix a regression since 5.19.0 about iommu=soft kernel parameter Julien ROBIN
@ 2026-10-08 10:27 ` Marek Szyprowski
0 siblings, 0 replies; 2+ messages in thread
From: Marek Szyprowski @ 2026-10-08 10:27 UTC (permalink / raw)
To: Julien ROBIN, linux-kernel
Cc: x86, Thomas Gleixner, Ingo Molnar, Borislav Petkov, Dave Hansen,
H. Peter Anvin, Aneesh Kumar K.V (Arm),
Christoph Hellwig, Konrad Rzeszutek Wilk, Boris Ostrovsky,
Thomas Gleixner, Ingo Molnar, Dave Hansen, H. Peter Anvin
On 04.10.2026 12:12, Julien ROBIN wrote:
> The following patch fixes a regression introduced since linux-5.19.0 about
> kernel parameter "iommu=soft" which, according to kernel-parameters.txt,
> is expected to enable the use of software bounce buffering (SWIOTLB) and to
> prevent the usage of an available hardware IOMMU.
>
> Since linux-5.19.0 however, iommu=soft doesn't prevent anymore the usage of
> hardware IOMMU implementations, and at least when an AMD GART IOMMU is
> available, SWIOTLB even gets disabled by "amd_gart_64.c" despite the
> iommu=soft parameter.
>
> The issue is fixed by adding an "x86_soft_iommu_only" variable which is set
> when the iommu=soft parameter is found and processed.
> If the variable is set, hardware IOMMU detection and allocation is skipped,
> as these detection and allocation functions could overwrite both
> x86_init.iommu.iommu_init and x86_platform.iommu_shutdown for later
> initialization and shutdown of these hardware IOMMU implementations.
>
> When the iommu=soft parameter isn't provided however, the behavior of the
> kernel is left untouched by this patch.
>
> Patch applies from 5.19.0 to 6.19.0, based on mainline as of 2026-02-12.
> Successfully tested this fix with iommu=soft parameter on linux-6.1.163 and
> linux-6.19.0 on a Gigabyte 990FXA-UD3 based computer.
>
> Tested the fix and iommu=soft parameter on linux-6.19 on these 4 machines:
> - Gigabyte 990FXA-UD3 motherboard based computer (AMD FX9590 CPU)
> - Asus TUF B550-PLUS motherboard based computer (AMD Ryzen 5 3700X CPU)
> - HP Victus 16-s1034nf / 8C9C laptop (AMD Ryzen 5 8645HS CPU)
> - Asus X550JK laptop (Intel Core i5-4200H CPU)
>
> On the 1st, this successfully disables AMD GART IOMMU and enables SWIOTLB.
> On the 2nd, this allows disabling AMD-Vi but SWIOTLB was already enabled.
> Same for the 3rd one.
> On the 4th, there was no hardware IOMMU enabled, and SWIOTLB was already
> enabled too: no resulting change for this laptop.
>
> Signed-off-by: Julien ROBIN <julien.robin28@free.fr>
I assume that this needs the following tags:
Fixes: 78013eaadf69 ("x86: remove the IOMMU table infrastructure")
Cc: stable@vger.kernel.org
It would be great if one of the x86 maintainers or developers involved into that
commit could comment on this patch. I would also like to know if it is safe to skip
gart, amd and intel iommu related calls.
> ---
> Changes since v1:
> - Added static keyword to 'x86_soft_iommu_only' (only used locally).
> ---
> arch/x86/kernel/pci-dma.c | 17 +++++++++++++----
> 1 file changed, 13 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kernel/pci-dma.c b/arch/x86/kernel/pci-dma.c
> index 75cf8f6ae8cd..c3dad499cd27 100644
> --- a/arch/x86/kernel/pci-dma.c
> +++ b/arch/x86/kernel/pci-dma.c
> @@ -34,6 +34,7 @@ int force_iommu __read_mostly = 0;
> int iommu_merge __read_mostly = 0;
>
> int no_iommu __read_mostly;
> +static int x86_soft_iommu_only __read_mostly;
> /* Set this to 1 if there is a HW IOMMU in the system */
> int iommu_detected __read_mostly = 0;
>
> @@ -100,9 +101,15 @@ void __init pci_iommu_alloc(void)
> return;
> }
> pci_swiotlb_detect();
> - gart_iommu_hole_init();
> - amd_iommu_detect();
> - detect_intel_iommu();
> +
> + if (x86_soft_iommu_only)
> + pr_info("PCI-DMA: skipping hardware IOMMU detection and allocation\n");
> + else {
> + gart_iommu_hole_init();
> + amd_iommu_detect();
> + detect_intel_iommu();
> + }
> +
> swiotlb_init(x86_swiotlb_enable, x86_swiotlb_flags);
> }
>
> @@ -149,8 +156,10 @@ static __init int iommu_setup(char *p)
> return 1;
> }
> #ifdef CONFIG_SWIOTLB
> - if (!strncmp(p, "soft", 4))
> + if (!strncmp(p, "soft", 4)) {
> x86_swiotlb_enable = true;
> + x86_soft_iommu_only = 1;
> + }
> #endif
> if (!strncmp(p, "pt", 2))
> iommu_set_default_passthrough(true);
>
Best regards
--
Marek Szyprowski, PhD
Samsung R&D Institute Poland
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