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

[Michael Kelley]

From: T.J. Mercier <tjmercier@xxxxxxxxxx> Sent: Monday, May 6, 2024 10:23 AM
> 
> 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

Note also that the documentation for swiotlb= in boot-options.rst is somewhat
out-of-date.  It doesn't have the optional second integer parameter to specify
the number of "areas" that have their own lock.  Perhaps that could be fixed
at the same time?

Michael