Re: [PATCH 0/1] swiotlb: Make IO_TLB_SEGSIZE Configurable

["Li, Hua Qian" <HuaQian.Li@siemens.com>]

On Fri, 2025-04-25 at 11:42 +0100, Robin Murphy wrote:
> On 2025-04-25 6:32 am, Li, Hua Qian wrote:
> > On Thu, 2025-04-24 at 13:58 +0100, Robin Murphy wrote:
> > > On 24/04/2025 6:12 am, Li, Hua Qian wrote:
> > > > On Tue, 2025-04-22 at 15:36 +0200, Marek Szyprowski wrote:
> > > > > On 22.04.2025 08:37, huaqian.li@siemens.com wrote:
> > > > > > From: Li Hua Qian <huaqian.li@siemens.com>
> > > > > >
> > > > > > This patchset introduces a change to make the
> > > > > > IO_TLB_SEGSIZE
> > > > > > parameter
> > > > > > configurable via a new kernel configuration option
> > > > > > (CONFIG_SWIOTLB_SEGSIZE).
> > > > > >
> > > > > > In certain applications, the default value of
> > > > > > IO_TLB_SEGSIZE
> > > > > > (128)
> > > > > > may
> > > > > > not be sufficient for memory allocation, leading to runtime
> > > > > > errors.
> > > > > > By
> > > > > > making this parameter configurable, users can adjust the
> > > > > > segment
> > > > > > size to
> > > > > > better suit their specific use cases, improving flexibility
> > > > > > and
> > > > > > system
> > > > > > stability.
> > > > >
> > > > > Could You elaborate a bit more what are those certain
> > > > > applications
> > > > > that
> > > > > require increasing IO_TLB_SEGSIZE? I'm not against it, but
> > > > > such
> > > > > change
> > > > > should be well justified and described, while the above
> > > > > cover-
> > > > > letter
> > > > > doesn't provide anything more than is written in the patch
> > > > > description.
> > > > Thank you for your feedback, Marek.
> > > >
> > > > To provide more context, one specific application that requires
> > > > increasing IO_TLB_SEGSIZE is the Hailo 8 PCIe AI card. This
> > > > card
> > > > uses
> > > > dma_alloc_coherent to allocate descriptor lists, as seen in the
> > > > Hailo
> > > > driver implementation here:
> > > > https://github.com/hailo-ai/hailort-drivers/blob/7161f9ee5918029bd4497f590003c2f87ec32507/linux/vdma/memory.c#L322
> > > > The maximum size (nslots) for these allocations can reach 160,
> > > > which
> > > > exceeds the current default value of IO_TLB_SEGSIZE (128).
> > > >
> > > > Since IO_TLB_SEGSIZE is defined as a constant in the kernel:
> > > >
> > > > `#define IO_TLB_SEGSIZE 128`
> > > >
> > > >
> > > > this limitation causes swiotlb_search_pool_area,
> > > > https://github.com/torvalds/linux/blame/v6.15-rc2/kernel/dma/swiotlb.c#L1085
> > > > ,
> > > > (or swiotlb_do_find_slots in older kernels) to fail when
> > > > attempting
> > > > to
> > > > allocate contiguous physical memory (CMA). This results in
> > > > runtime
> > > > errors and prevents the Hailo 8 card from functioning correctly
> > > > in
> > > > certain configurations.
> > >
> > > Hmm, dma_alloc_coherent() should really not be trying to allocate
> > > from
> > > SWIOTLB in the first place - how is that happening?
> > >
> > > If you're using restricted DMA for a device which wants
> > > significant
> > > coherent allocations, then it wants to have it's own shared-dma-
> > > pool
> > > for
> > > those *as well* as the restricted-dma-pool for bouncing streaming
> > > DMA.
> > >
> > > Thanks,
> > > Robin.
> >
> > Hi Robin,
> >
> > Regarding the specific Hailo Card case, the issue arises due
> > to the capabilities of certain SoCs or CPUs. For example, many
> > K3 SoCs lack an IOMMU, which is typically used to isolate the
> > system against DMA-based attacks of external PCI devices.
> >
> > Taking the TI AM65 as an example, it doesn't have an IOMMU, but
> > instead includes a Peripheral Virtualization Unit (PVU). The
> > PVU provides functionality similar to an IOMMU and is used to
> > isolate PCI devices from the Linux host, and the SWIOTLB is
> > used to manp all DMA buffers from a static memory carve-out.
>
> And as I said, if you want to support general coherent allocations
> then
> you should use part of that carveout for a regular coherent DMA pool.
> The restricted pool is only intended for streaming DMA -
> swiotlb_alloc()
> is only meant as a convenience fallback for the kind of devices which
> mostly do streaming DMA but make one or two small coherent
> allocations
> from a suitable context. It does not work for *all* valid usage of
> dma_alloc_attrs(), and if you want to do this for arbitrary PCI
> devices
> then you almost certainly *do* need to be able to support drivers
> which
> make allocations in atomic context.
>
> Thanks,
> Robin.
Thank you for your detailed explanation, Robin. I understand your
points and agree with your suggestions. Based on this, I will conclude
this upstreaming patchset here. Thank you again for your guiidance and
support.

Best Regards,
Li Hua Qian
>
> > You can find more details and background information here:
> > https://lore.kernel.org/all/20250422061406.112539-1-huaqian.li@siemens.com/
> >
> > >
> > > > By making IO_TLB_SEGSIZE configurable via a kernel
> > > > configuration
> > > > option
> > > > (CONFIG_SWIOTLB_SEGSIZE), users can adjust the segment size to
> > > > accommodate such use cases. This change improves flexibility
> > > > and
> > > > ensures that systems can be tailored to meet the requirements
> > > > of
> > > > specific hardware, such as the Hailo 8 PCIe AI card, without
> > > > requiring
> > > > kernel source modifications.
> > > >
> > > > I hope this example clarifies the need for this change. Please
> > > > let
> > > > me
> > > > know if further details or additional examples are required.
> > > >
> > > > Best Regards,
> > > > Li Hua Qian
> > > > >
> > > > >
> > > > > > Li Hua Qian (1):
> > > > > >      swiotlb: Make IO_TLB_SEGSIZE configurable
> > > > > >
> > > > > >     include/linux/swiotlb.h | 2 +-
> > > > > >     kernel/dma/Kconfig      | 7 +++++++
> > > > > >     2 files changed, 8 insertions(+), 1 deletion(-)
> > > > > >
> > > > > Best regards
> > > >
> >
> > --
> > Hua Qian Li
> > Siemens AG
> > http://www.siemens.com/
>

--
Hua Qian Li
Siemens AG
http://www.siemens.com/