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Lu Baolu d2138abc8f iommu/vt-d: Clear Present bit before tearing down context entry 
[ Upstream commit c1e4f1dccb ]

When tearing down a context entry, the current implementation zeros the
entire 128-bit entry using multiple 64-bit writes. This creates a window
where the hardware can fetch a "torn" entry — where some fields are
already zeroed while the 'Present' bit is still set — leading to
unpredictable behavior or spurious faults.

While x86 provides strong write ordering, the compiler may reorder writes
to the two 64-bit halves of the context entry. Even without compiler
reordering, the hardware fetch is not guaranteed to be atomic with
respect to multiple CPU writes.

Align with the "Guidance to Software for Invalidations" in the VT-d spec
(Section 6.5.3.3) by implementing the recommended ownership handshake:

1. Clear only the 'Present' (P) bit of the context entry first to
   signal the transition of ownership from hardware to software.
2. Use dma_wmb() to ensure the cleared bit is visible to the IOMMU.
3. Perform the required cache and context-cache invalidation to ensure
   hardware no longer has cached references to the entry.
4. Fully zero out the entry only after the invalidation is complete.

Also, add a dma_wmb() to context_set_present() to ensure the entry
is fully initialized before the 'Present' bit becomes visible.

Fixes: ba39592764 ("Intel IOMMU: Intel IOMMU driver")
Reported-by: Dmytro Maluka <dmaluka@chromium.org>
Closes: https://lore.kernel.org/all/aTG7gc7I5wExai3S@google.com/
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Dmytro Maluka <dmaluka@chromium.org>
Reviewed-by: Samiullah Khawaja <skhawaja@google.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Link: https://lore.kernel.org/r/20260120061816.2132558-3-baolu.lu@linux.intel.com
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
History 2026-02-26 14:59:18 -08:00
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README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.