76901e56fb
Explicitly check if a NX huge page is disallowed when determining if a
page fault needs to be forced to use a smaller sized page. KVM currently
assumes that the NX huge page mitigation is the only scenario where KVM
will force a shadow page instead of a huge page, and so unnecessarily
keeps an existing shadow page instead of replacing it with a huge page.
Any scenario that causes KVM to zap leaf SPTEs may result in having a SP
that can be made huge without violating the NX huge page mitigation.
E.g. prior to commit 5ba7c4c6d1c7 ("KVM: x86/MMU: Zap non-leaf SPTEs when
disabling dirty logging"), KVM would keep shadow pages after disabling
dirty logging due to a live migration being canceled, resulting in
degraded performance due to running with 4kb pages instead of huge pages.
Although the dirty logging case is "fixed", that fix is coincidental,
i.e. is an implementation detail, and there are other scenarios where KVM
will zap leaf SPTEs. E.g. zapping leaf SPTEs in response to a host page
migration (mmu_notifier invalidation) to create a huge page would yield a
similar result; KVM would see the shadow-present non-leaf SPTE and assume
a huge page is disallowed.
Fixes: b8e8c8303ff2 ("kvm: mmu: ITLB_MULTIHIT mitigation")
Reviewed-by: Ben Gardon <bgardon@google.com>
Reviewed-by: David Matlack <dmatlack@google.com>
Signed-off-by: Mingwei Zhang <mizhang@google.com>
[sean: use spte_to_child_sp(), massage changelog, fold into if-statement]
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>
Message-Id: <20221019165618.927057-8-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.