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If a vCPU is scheduling out and not in WFI emulation, it is highly likely it will get scheduled again soon and reuse the MMU it had before. Dropping the MMU at vcpu_put() can have some unfortunate consequences, as the MMU could get reclaimed and used in a different context, forcing another 'cold start' on an otherwise active MMU. Avoid that altogether by keeping a reference on the MMU if the vCPU is scheduling out, ensuring that another vCPU cannot reclaim it while the current vCPU is away. Since there are more MMUs than vCPUs, this does not affect the guarantee that an unused MMU is available at any time. Furthermore, this makes the vcpu->arch.hw_mmu ~stable in preemptible code, at least for where it matters in the stage-2 abort path. Yes, the MMU can change across WFI emulation, but there isn't even a use case where this would matter. Signed-off-by: Oliver Upton <oliver.upton@linux.dev> Link: https://lore.kernel.org/r/20241007233028.2236133-2-oliver.upton@linux.dev Signed-off-by: Marc Zyngier <maz@kernel.org>
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.