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When allocating a new TDP MMU root, check for a usable root while holding mmu_lock for read and only acquire mmu_lock for write if a new root needs to be created. There is no need to serialize other MMU operations if a vCPU is simply grabbing a reference to an existing root, holding mmu_lock for write is "necessary" (spoiler alert, it's not strictly necessary) only to ensure KVM doesn't end up with duplicate roots. Allowing vCPUs to get "new" roots in parallel is beneficial to VM boot and to setups that frequently delete memslots, i.e. which force all vCPUs to reload all roots. Link: https://lore.kernel.org/r/20240111020048.844847-7-seanjc@google.com Signed-off-by: Sean Christopherson <seanjc@google.com>
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.