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Date: Fri, 17 May 2024 17:38:02 +0000
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Subject: [PATCH v3 1/2] doc: swiotlb: iommu/dma: Clarify swiotlb=force option
 applies only to dma-direct
From: "T.J. Mercier" <tjmercier@google.com>
To: tjmercier@google.com, Jonathan Corbet <corbet@lwn.net>, 
	Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>, 
	Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org, 
	"H. Peter Anvin" <hpa@zytor.com>
Cc: iommu@lists.linux.dev, isaacmanjarres@google.com, robin.murphy@arm.com, 
	will@kernel.org, joro@8bytes.org, Petr Tesarik <petr@tesarici.cz>, 
	linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org
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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=force 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>
Reviewed-by: Petr Tesarik <petr@tesarici.cz>
---
 Documentation/admin-guide/kernel-parameters.txt | 1 +
 Documentation/arch/x86/x86_64/boot-options.rst  | 3 +++
 2 files changed, 4 insertions(+)

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..a37139d1752f 100644
--- a/Documentation/arch/x86/x86_64/boot-options.rst
+++ b/Documentation/arch/x86/x86_64/boot-options.rst
@@ -292,6 +292,9 @@ implementation:
         Prereserve that many 2K slots for the software IO bounce buffering.
       force
         Force all IO through the software TLB.
+        Hardware IOMMU implementations can use SWIOTLB bounce buffering in
+        some circumstances, but they cannot be forced to always use them, so
+        this option only has an effect when no hardware IOMMU is involved.
       noforce
         Do not initialize the software TLB.
 
-- 
2.45.0.rc1.225.g2a3ae87e7f-goog