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arm64: errata: work around NVIDIA Olympus device store/load ordering

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On systems with NVIDIA Olympus cores, a Device-nGnR* load can be
observed by a peripheral before an older, non-overlapping Device-nGnR*
store to the same peripheral. This breaks the program-order guarantee
that software expects for Device-nGnR* accesses and can leave a
peripheral in an incorrect state.

The erratum can occur only when all of the following apply:

  - A PE executes a Device-nGnR* store followed by a younger
    Device-nGnR* load.
  - The store is not a store-release.
  - The accesses target the same peripheral and do not overlap in bytes.
  - There is at most one intervening Device-nGnR* store in program
    order, and there are no intervening Device-nGnR* loads.
  - There is no DSB or full DMB between the store and the load.
  - Specific microarchitectural and timing conditions occur.

Insert a DMB OSH immediately before each raw MMIO load on affected CPUs.
As a full barrier, DMB OSH orders the older Device store before the
younger Device load and prevents the erroneous observation.

Add the barrier directly to the __raw_read*() helpers, independently of
the existing device-load-acquire alternative. On affected CPUs this adds
one DMB OSH per raw MMIO load, including each load used by
memcpy_fromio(). On unaffected CPUs the alternative remains a NOP.

Co-developed-by: Vikram Sethi <vsethi@nvidia.com>
Signed-off-by: Vikram Sethi <vsethi@nvidia.com>
Signed-off-by: Shanker Donthineni <sdonthineni@nvidia.com>
Link: https://lore.kernel.org/all/akPQ8F3OgER621UP@willie-the-truck/
Reviewed-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>
This commit is contained in:
Shanker Donthineni 2026-07-15 15:48:56 -05:00 • committed by Will Deacon
commit 12aab25ca5
5 changed files with 45 additions and 4 deletions
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