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@xxxxxxx [2] https://lore.kernel.org/all/20190906061452.30791-1-baolu.lu@xxxxxxxxxxxxxxx/ Signed-off-by: T.J. Mercier <tjmercier@xxxxxxxxxx> Reviewed-by: Petr Tesarik <petr@xxxxxxxxxxx> --- 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
