[PATCH] swiotlb: iommu/dma: Clarify swiotlb options apply only to dma-direct

["T.J. Mercier" <tjmercier@google.com>]

IOMMU implementations now sometimes bounce memory through SWIOTLB to
achieve cacheline alignment [1], or prevent DMA attacks by untrusted
devices [2]. These uses of SWIOTLB differ conceptually from historical
use which was a solution to the problem of device addressing
limitations that prevent DMA to some portion of system memory
(typically beyond 4 GiB). IOMMUs also solve the problem of device
addressing limitations and therefore eliminate the need for SWIOTLB for
that purpose. However as mentioned, IOMMUs can use SWIOTLB for other
purposes.

The swiotlb kernel command line parameter does not impact IOMMU related
use of SWIOTLB, and that is intentional. IOMMUs cannot be forced to use
SWIOTLB for all buffers. Update the documentation for the swiotlb
parameter to clarify that SWIOTLB use can only be forced in scenarios
where an IOMMU is not involved.

[1] https://lore.kernel.org/all/20230612153201.554742-16-catalin.marinas@arm.com
[2] https://lore.kernel.org/all/20190906061452.30791-1-baolu.lu@linux.intel.com/
Signed-off-by: T.J. Mercier <tjmercier@google.com>
---
 Documentation/admin-guide/kernel-parameters.txt | 1 +
 Documentation/arch/x86/x86_64/boot-options.rst  | 2 +-
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 213d0719e2b7..84c582ac246c 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -6486,6 +6486,7 @@
 				 to a power of 2.
 			force -- force using of bounce buffers even if they
 			         wouldn't be automatically used by the kernel
+			         where a hardware IOMMU is not involved
 			noforce -- Never use bounce buffers (for debugging)
 
 	switches=	[HW,M68k,EARLY]
diff --git a/Documentation/arch/x86/x86_64/boot-options.rst b/Documentation/arch/x86/x86_64/boot-options.rst
index 137432d34109..066b4bc81583 100644
--- a/Documentation/arch/x86/x86_64/boot-options.rst
+++ b/Documentation/arch/x86/x86_64/boot-options.rst
@@ -285,7 +285,7 @@ iommu options only relevant to the AMD GART hardware IOMMU:
       Always panic when IOMMU overflows.
 
 iommu options only relevant to the software bounce buffering (SWIOTLB) IOMMU
-implementation:
+implementation where a hardware IOMMU is not involved:
 
     swiotlb=<slots>[,force,noforce]
       <slots>
-- 
2.45.0.rc1.225.g2a3ae87e7f-goog