[PATCH v2 0/7] Allow dynamic allocation of software IO TLB bounce buffers

[Mike Lothian]

On Wed, 19 Apr 2023 at 11:05, Petr Tesarik <petrtesarik at huaweicloud.com> wrote:
>
> From: Petr Tesarik <petr.tesarik.ext at huawei.com>
>
> The goal of my work is to provide more flexibility in the sizing of
> SWIOTLB.
>
> The software IO TLB was designed with these assumptions:
>
> 1. It would not be used much, especially on 64-bit systems.
> 2. A small fixed memory area (64 MiB by default) is sufficient to
>    handle the few cases which require a bounce buffer.
> 3. 64 MiB is little enough that it has no impact on the rest of the
>    system.
>
> First, if SEV is active, all DMA must be done through shared
> unencrypted pages, and SWIOTLB is used to make this happen without
> changing device drivers. The software IO TLB size is increased to
> 6% of total memory in sev_setup_arch(), but that is more of an
> approximation. The actual requirements may vary depending on the
> amount of I/O and which drivers are used. These factors may not be
> know at boot time, i.e. when SWIOTLB is allocated.
>
> Second, other colleagues have noticed that they can reliably get
> rid of occasional OOM kills on an Arm embedded device by reducing
> the SWIOTLB size. This can be achieved with a kernel parameter, but
> determining the right value puts additional burden on pre-release
> testing, which could be avoided if SWIOTLB is allocated small and
> grows only when necessary.
>
> Changes from v1-devel-v7:
> - Add comments to acquire/release barriers
> - Fix whitespace issues reported by checkpatch.pl
>
> Changes from v1-devel-v6:
> - Provide long description of functions
> - Fix kernel-doc (Returns: to Return:)
> - Rename __lookup_dyn_slot() to lookup_dyn_slot_locked()
>
> Changes from RFC:
> - Track dynamic buffers per device instead of per swiotlb
> - Use a linked list instead of a maple tree
> - Move initialization of swiotlb fields of struct device to a
>   helper function
> - Rename __lookup_dyn_slot() to lookup_dyn_slot_locked()
> - Introduce per-device flag if dynamic buffers are in use
> - Add one more user of DMA_ATTR_MAY_SLEEP
> - Add kernel-doc comments for new (and some old) code
> - Properly escape '*' in dma-attributes.rst
>
> Petr Tesarik (7):
>   swiotlb: Use a helper to initialize swiotlb fields in struct device
>   swiotlb: Move code around in preparation for dynamic bounce buffers
>   dma-mapping: introduce the DMA_ATTR_MAY_SLEEP attribute
>   swiotlb: Dynamically allocated bounce buffers
>   swiotlb: Add a boot option to enable dynamic bounce buffers
>   drm: Use DMA_ATTR_MAY_SLEEP from process context
>   swiotlb: per-device flag if there are dynamically allocated buffers
>
>  .../admin-guide/kernel-parameters.txt         |   6 +-
>  Documentation/core-api/dma-attributes.rst     |  10 +
>  drivers/base/core.c                           |   4 +-
>  drivers/gpu/drm/drm_gem_shmem_helper.c        |   2 +-
>  drivers/gpu/drm/drm_prime.c                   |   2 +-
>  include/linux/device.h                        |  12 +
>  include/linux/dma-mapping.h                   |   6 +
>  include/linux/swiotlb.h                       |  54 ++-
>  kernel/dma/swiotlb.c                          | 382 ++++++++++++++++--
>  9 files changed, 443 insertions(+), 35 deletions(-)
>
> --
> 2.25.1
>

Hi

Is this a potential fix for
https://bugzilla.kernel.org/show_bug.cgi?id=217310 where I'm manually
setting bigger buffers to keep my wifi working?

Thanks

Mike