4f510c8bb1
We are dropping A/D bits (and W bits) in the TDP MMU. Even if mmu_lock is held for write, as volatile SPTEs can be written by other tasks/vCPUs outside of mmu_lock. Attempting to prove that bug exposed another notable goof, which has been lurking for a decade, give or take: KVM treats _all_ MMU-writable SPTEs as volatile, even though KVM never clears WRITABLE outside of MMU lock. As a result, the legacy MMU (and the TDP MMU if not fixed) uses XCHG to update writable SPTEs. The fix does not seem to have an easily-measurable affect on performance; page faults are so slow that wasting even a few hundred cycles is dwarfed by the base cost.
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.