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Set the Accessed bit when making a "new" SPTE during SPTE synchronization, as _clearing_ the Accessed bit is counter-productive, and even if the Accessed bit wasn't set in the old SPTE, odds are very good the guest will access the page in the near future, as the most common case where KVM synchronizes a shadow-present SPTE is when the guest is making the gPTE read-only for Copy-on-Write (CoW). Preserving the Accessed bit will allow dropping the logic that propagates the Accessed bit to the underlying struct page when overwriting an existing SPTE, without undue risk of regressing page aging. Note, KVM's current behavior is very deliberate, as SPTE synchronization was the only "speculative" access type as of commit947da53830("KVM: MMU: Set the accessed bit on non-speculative shadow ptes"). But, much has changed since 2008, and more changes are on the horizon. Spurious clearing of the Accessed (and Dirty) was mitigated by commite6722d9211("KVM: x86/mmu: Reduce the update to the spte in FNAME(sync_spte)"), which changed FNAME(sync_spte) to only overwrite SPTEs if the protections are actually changing. I.e. KVM is already preserving Accessed information for SPTEs that aren't dropping protections. And with the aforementioned future change to NOT mark the page/folio as accessed, KVM's SPTEs will become the "source of truth" so to speak, in which case clearing the Accessed bit outside of page aging becomes very undesirable. Suggested-by: Yan Zhao <yan.y.zhao@intel.com> Signed-off-by: Sean Christopherson <seanjc@google.com> Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Message-ID: <20241010182427.1434605-8-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.