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If mmu_lock is held for write, don't bother setting !PRESENT SPTEs to REMOVED_SPTE when recursively zapping SPTEs as part of shadow page removal. The concurrent write protections provided by REMOVED_SPTE are not needed, there are no backing page side effects to record, and MMIO SPTEs can be left as is since they are protected by the memslot generation, not by ensuring that the MMIO SPTE is unreachable (which is racy with respect to lockless walks regardless of zapping behavior). Skipping !PRESENT drastically reduces the number of updates needed to tear down sparsely populated MMUs, e.g. when tearing down a 6gb VM that didn't touch much memory, 6929/7168 (~96.6%) of SPTEs were '0' and could be skipped. Avoiding the write itself is likely close to a wash, but avoiding __handle_changed_spte() is a clear-cut win as that involves saving and restoring all non-volatile GPRs (it's a subtly big function), as well as several conditional branches before bailing out. Cc: Ben Gardon <bgardon@google.com> Signed-off-by: Sean Christopherson <seanjc@google.com> Message-Id: <20210310003029.1250571-1-seanjc@google.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.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.