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

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

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;
-- 
2.43.0