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The MPIDR_EL1 register contains a unique value that identifies the CPU. The only problem with it is that it is stupidly large (32 bits, once the useless stuff is removed). Trying to obtain a vcpu from an MPIDR value is a fairly common, yet costly operation: we iterate over all the vcpus until we find the correct one. While this is cheap for small VMs, it is pretty expensive on large ones, specially if you are trying to get to the one that's at the end of the list... In order to help with this, it is important to realise that the MPIDR values are actually structured, and that implementations tend to use a small number of significant bits in the 32bit space. We can use this fact to our advantage by computing a small hash table that uses the "compression" of the significant MPIDR bits as an index, giving us the vcpu index as a result. Given that the MPIDR values can be supplied by userspace, and that an evil VMM could decide to make *all* bits significant, resulting in a 4G-entry table, we only use this method if the resulting table fits in a single page. Otherwise, we fallback to the good old iterative method. Nothing uses that table just yet, but keep your eyes peeled. Reviewed-by: Joey Gouly <joey.gouly@arm.com> Reviewed-by: Zenghui Yu <yuzenghui@huawei.com> Tested-by: Joey Gouly <joey.gouly@arm.com> Tested-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com> Signed-off-by: Marc Zyngier <maz@kernel.org> Link: https://lore.kernel.org/r/20230927090911.3355209-9-maz@kernel.org Signed-off-by: Oliver Upton <oliver.upton@linux.dev> |
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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 Restructured Text 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.