On Fri, 3 May 2024 18:35:26 +0000 "T.J. Mercier" <tjmercier@xxxxxxxxxx> wrote: > 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@xxxxxxx > [2] https://lore.kernel.org/all/20190906061452.30791-1-baolu.lu@xxxxxxxxxxxxxxx/ > Signed-off-by: T.J. Mercier <tjmercier@xxxxxxxxxx> > --- > 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] Yes, this part is correct. SWIOTLB cannot be forced if there is an IOMMU. > 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> But this part needs some improvement. The "swiotlb" option is not entirely ignored if there is a hardware IOMMU. For example, the size of the SWIOTLB can be adjusted using "swiotlb=<slots>", and since SWIOTLB can be used by IOMMUs for other purposes (as you correctly note in the commit message), this setting is relevant even where a hardware IOMMU is involved. Petr T
