Re: [RFC PATCH] x86/pci-dma: add "any" keyword to swiotlb= kernel parameter

[Aakarsh Jain <aakarsh.jain@oss.qualcomm.com>]

On 7/8/2026 5:12 PM, Aakarsh Jain wrote:
> CoCo guests (AMD SEV-SNP, Intel TDX) require large swiotlb pools for
> streaming DMA workloads such as high-speed NIC and AI accelerator
> inference. The existing swiotlb pool allocator restricts placement to
> low memory (below 4GB by default), capping usable pool size at ~1GB even
> when a larger pool is requested via swiotlb=<nslabs>.
> 
> The SWIOTLB_ANY flag already exists to lift this restriction, and
> swiotlb_init_remap() already handles it correctly via the flags
> parameter (see CONFIG_SWIOTLB_DYNAMIC path: io_tlb_default_mem.phys_limit
> is set to virt_to_phys(high_memory-1) when SWIOTLB_ANY is set).
> 
> However, there is no way to set SWIOTLB_ANY from the command line. The
> only existing mechanism was via arch-specific code (e.g. powerpc SVM sets
> SWIOTLB_ANY in pci_iommu_init). x86 CoCo guests have no such path.
> 
> After Aneesh  series ("dma-mapping: Track shared DMA state through
> direct, pool and swiotlb paths", https://patchwork.kernel.org/project/linux-arm-kernel/cover/20260701054926.825925-1-aneesh.kumar@kernel.org/)
> removes SWIOTLB_FORCE, x86 pci_swiotlb_detect() leaves x86_swiotlb_flags = 0 for
> CoCo guests. The pool falls back to low memory and caps at ~1GB:
> 
>    Without "any": pool at 0x35a9c000 (~900MB, below 4GB boundary)
>    With    "any": pool at 0x1df9c00000 (~120GB, anywhere in RAM)
>    [Tested on AMD SEV-SNP guest, swiotlb=4194304]
> 
> Add "any" as a new keyword to the swiotlb= kernel parameter. This is an
> explicit, opt-in mechanism that sets SWIOTLB_ANY for the default pool at
> boot time, without touching any arch-specific code.
> 
> Devices with 32-bit DMA masks are not affected, they still use the normal
> low-memory bounce buffer path. The "any" option is only meaningful for
> workloads where all active DMA devices have 64-bit masks.
> 
> Signed-off-by: Aakarsh Jain <aakarsh.jain@oss.qualcomm.com>
> ---
>   Documentation/admin-guide/kernel-parameters.txt | 5 ++++-
>   kernel/dma/swiotlb.c                            | 5 +++++
>   2 files changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index b5493a7f8f22..8a1fccbd9b25 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -7477,7 +7477,7 @@ Kernel parameters
>   			Execution Facility on pSeries.
>   
>   	swiotlb=	[ARM,PPC,MIPS,X86,S390,EARLY]
> -			Format: { <int> [,<int>] | force | noforce }
> +			Format: { <int> [,<int>] | force | noforce | any}
>   			<int> -- Number of I/O TLB slabs
>   			<int> -- Second integer after comma. Number of swiotlb
>   				 areas with their own lock. Will be rounded up
> @@ -7485,6 +7485,9 @@ Kernel parameters
>   			force -- force using of bounce buffers even if they
>   			         wouldn't be automatically used by the kernel
>   			noforce -- Never use bounce buffers (for debugging)
> +			any --  Allow the swiotlb pool to be placed anywhere in
> +				system RAM, lifting the default low-memory (4GB)
> +				restriction.
>   
>   	switches=	[HW,M68k,EARLY]
>   
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index 1abd3e6146f4..34773ae7c770 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -80,6 +80,7 @@ struct io_tlb_slot {
>   
>   static bool swiotlb_force_bounce;
>   static bool swiotlb_force_disable;
> +static unsigned int swiotlb_param_flags __initdata;
>   
>   #ifdef CONFIG_SWIOTLB_DYNAMIC
>   
> @@ -198,6 +199,8 @@ setup_io_tlb_npages(char *str)
>   		swiotlb_force_bounce = true;
>   	else if (!strcmp(str, "noforce"))
>   		swiotlb_force_disable = true;
> +	else if (!strcmp(str, "any"))
> +		swiotlb_param_flags |= SWIOTLB_ANY;
>   
>   	return 0;
>   }
> @@ -445,6 +448,8 @@ int swiotlb_init_late(size_t size, gfp_t gfp_mask,
>   
>   	io_tlb_default_mem.force_bounce = swiotlb_force_bounce;
>   
> +	flags |= swiotlb_param_flags;
> +
>   #ifdef CONFIG_SWIOTLB_DYNAMIC
>   	if (!remap)
>   		io_tlb_default_mem.can_grow = true;


One correction to the commit message:

The testing was performed with:

   swiotlb=4194304,any

and not:

   swiotlb=4194304

The larger swiotlb pool allocation above the 4GB boundary was observed
with the "any" option enabled.


 From kernel dmesg logs:
root@ubuntu:/home/ubuntu# dmesg | grep -i sev
[   21.191917] Memory Encryption Features active: AMD SEV SEV-ES SEV-SNP
[   21.192883] SEV: Status: SEV SEV-ES SEV-SNP
[   21.401897] SEV: APIC: wakeup_secondary_cpu() replaced with 
wakeup_cpu_via_vmgexit()
[   22.117267] SEV: Using SNP CPUID table, 28 entries present.
[   22.117884] SEV: SNP running at VMPL0.
[   24.164260] SEV: SNP guest platform devices initialized.
[   28.815142] systemd[1]: Detected confidential virtualization sev-snp.
[   30.063831] sev-guest sev-guest: Initialized SEV guest driver (using 
VMPCK0 communication key)
root@ubuntu:/home/ubuntu# dmesg | grep -i "IO TLB"
[    6.743607] software IO TLB: area num 4.
[   24.164169] software IO TLB: mapped [mem 
0x0000001df9c00000-0x0000001ff9c00000] (8192MB)
[   24.331266] software IO TLB: Memory encryption is active and system 
is using DMA bounce buffers

Thanks,
Aakarsh