[PATCH 0/4] KVM: x86/mmu: Add auto-prefetch to the TDP MMU

[Sean Christopherson]

Add auto-prefetch to the TDP MMU, where "auto-prefetch" means the existing
prefetch in the shadow MMU where KVM grabs the surrounding ~8 SPTEs on a page
fault, when possible.

Rather than implement yet another custom solution, reuse the core functionality
from KVM_PRE_FAULT_MEMORY.  Doing so has several benefits, and a few drawbacks.
The big benefits are code reuse, reduced maintainence burden improved test
coverage (auto-prefetching exposed a variety of pre-existing issues in the
pre-fault code), and support for any backing store (the shadow MMU's approach
only works for fast-GUP, i.e. only for refcounted struct page memory).

The big drawback is that using the pre-fault code is "heavier" than the custom
prefetching code in the shadow MMU.  In a nutshell, this approach can't achieve
the same performance uplift at least not without additional tweaking (which may
or may not be acceptable; I genuinely haven't thought at all about how we could
close the gap, mainly because it's not clear that we *need* to close the gap).

There are a few subjective decisions in here.  The main one I want to call out
is that I chose to treat async #PF prefetching as manual/explicit prefetching,
versus automatic prefetching.  Manual prefetching will do more invasive
operations like punch through dirty logging and wait for I/O, whereas automatic
prefetching avoids spurious dirty logging and tries to avoid waiting on "major"
faults.  This series preserves the existing behavior, but I spent enough time
waffling on whether or not that was the right choice that I wanted to call it
out here.

Sean Christopherson (4):
  KVM: x86/mmu: Rework gfn_to_memslot_dirty_bitmap() into auto-prefetch
    helper
  KVM: x86/mmu: Differentiate between manual and automatic prefetching
  KVM: x86/mmu: Add automatic prefaulting/prefetching for TDP MMU faults
  KVM: x86/mmu: Use the generic auto-prefaulting to do prefetch for
    direct MMUs

 arch/x86/kvm/mmu/mmu.c          | 156 +++++++++++++++++++-------------
 arch/x86/kvm/mmu/mmu_internal.h |   1 +
 arch/x86/kvm/mmu/paging_tmpl.h  |   2 +-
 3 files changed, 97 insertions(+), 62 deletions(-)


base-commit: 7e60162b16f9da0d2fa83da5c1723b6853a0ac1d
-- 
2.56.0.313.g2479322e68-goog