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a577509095
Use MMU read lock to recover TDP MMU NX huge pages. To prevent concurrent modification of the list of potential huge pages, iterate over the list under tdp_mmu_pages_lock protection and unaccount the page before dropping the lock. Zapping under MMU read lock unblocks vCPUs which are waiting for MMU read lock, which solves a guest jitter issue on Windows VMs which were observing an increase in network latency. Do not zap an SPTE if: - The SPTE is a root page. - The SPTE does not point at the SP's page table. If the SPTE does not point at the SP's page table, then something else has change the SPTE, so KVM cannot safely zap it. Warn if zapping SPTE fails and current SPTE is still pointing to same page table, as it should be impossible for the CMPXCHG to fail due to all other write scenarios being mutually exclusive. There is always a race between dirty logging, vCPU faults, and NX huge page recovery for backing a gfn by an NX huge page or an executable small page. Unaccounting sooner during the list traversal increases the window of that race, but functionally, it is okay. Accounting doesn't protect against iTLB multi-hit bug, it is there purely to prevent KVM from bouncing a gfn between two page sizes. The only downside is that a vCPU will end up doing more work in tearing down all the child SPTEs. This should be a very rare race. Suggested-by: Sean Christopherson <seanjc@google.com> Signed-off-by: Vipin Sharma <vipinsh@google.com> Co-developed-by: James Houghton <jthoughton@google.com> Signed-off-by: James Houghton <jthoughton@google.com> Link: https://lore.kernel.org/r/20250707224720.4016504-4-jthoughton@google.com [sean: clean up kvm_mmu_sp_dirty_logging_enabled() and the changelog] 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 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.