KVM: x86/mmu: Use a dedicated bit to track shadow/MMU-present SPTEs

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Introduce MMU_PRESENT to explicitly track which SPTEs are "present" from
the MMU's perspective.  Checking for shadow-present SPTEs is a very
common operation for the MMU, particularly in hot paths such as page
faults.  With the addition of "removed" SPTEs for the TDP MMU,
identifying shadow-present SPTEs is quite costly especially since it
requires checking multiple 64-bit values.

On 64-bit KVM, this reduces the footprint of kvm.ko's .text by ~2k bytes.
On 32-bit KVM, this increases the footprint by ~200 bytes, but only
because gcc now inlines several more MMU helpers, e.g. drop_parent_pte().

We now need to drop bit 11, used for the MMU_PRESENT flag, from
the set of bits used to store the generation number in MMIO SPTEs.
Otherwise MMIO SPTEs with bit 11 set would get false positives for
is_shadow_present_spte() and lead to a variety of fireworks, from oopses
to likely hangs of the host kernel.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210225204749.1512652-21-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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Sean Christopherson 2021-02-25 12:47:45 -08:00 • committed by Paolo Bonzini

parent 613a3f3797

commit edea7c4fc2

2 changed files with 21 additions and 10 deletions

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8

arch/x86/kvm/mmu/spte.c

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23

arch/x86/kvm/mmu/spte.h

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