Re: [PATCH rc v6 6/7] iommu/arm-smmu-v3: Skip RMR bypass for kdump adoption

[Pranjal Shrivastava]

On Wed, May 20, 2026 at 10:03:23AM -0700, Nicolin Chen wrote:
> RMR bypass STEs are installed during SMMUv3 probe for StreamIDs listed by
> IORT RMR nodes. A normal boot switches the driver to a fresh stream table
> whose initial STEs abort, so those RMR SIDs need bypass entries before it
> becomes live. This preserves firmware/guest-owned traffic, including vSMMU
> guest MSI cases built around RMR-described SIDs.
> 
> ARM_SMMU_OPT_KDUMP_ADOPT is the opposite case: the driver keeps SMMUEN set
> and adopts the crashed kernel's stream table, so RMR SIDs already have the
> only translation state known to be safe for active in-flight DMA. Replacing
> an adopted STE with bypass can turn translated DMA into physical DMA, then
> point it at the wrong memory.
> 
> arm_smmu_make_bypass_ste() also rewrites the STE in place after clearing it
> first. While the table is live, a concurrent hardware STE fetch can observe
> V=0 or mixed old/new state.
> 
> Leaving the adopted STE unmodified keeps the kdump kernel using the crashed
> kernel's translation. That gives the endpoint driver a chance to probe and
> quiesce the device.
> 
> If the old STE was already abort or invalid, installing bypass would create
> new DMA permission; leaving it alone is a safer failure mode. Later domain
> setup still gets the RMR direct mappings through the reserved-region path.
> 
> Fixes: b63b3439b856 ("iommu/arm-smmu-v3: Abort all transactions if SMMU is enabled in kdump kernel")
> Cc: stable@xxxxxxxxxxxxxxx # v6.12+
> Assisted-by: Codex:gpt-5.5
> Signed-off-by: Nicolin Chen <nicolinc@xxxxxxxxxx>
 
Reviewed-by: Pranjal Shrivastava <praan@xxxxxxxxxx>

Thanks,
Praan