On Sat, May 4, 2024 at 1:16 AM Petr Tesařík <petr@xxxxxxxxxxx> wrote: > > 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 Thanks. I think I should also update the commit message: "The swiotlb=force kernel command line parameter does not impact IOMMU related use of SWIOTLB" and title: "Clarify swiotlb=force option applies only to dma-direct" As for fixing boot-options.txt, I think it makes the most sense to expand on just the force option rather than the section summary like above: 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
