[RFC PATCH 0/6] Pass memslot around during page fault handling

* [RFC PATCH 0/6] Pass memslot around during page fault handling
@ 2021-08-13 20:34 David Matlack
  2021-08-13 20:34 ` [RFC PATCH 1/6] KVM: x86/mmu: Rename try_async_pf to kvm_faultin_pfn in comment David Matlack
                   ` (6 more replies)
  0 siblings, 7 replies; 18+ messages in thread
From: David Matlack @ 2021-08-13 20:34 UTC (permalink / raw)
  To: Paolo Bonzini
  Cc: kvm, Ben Gardon, Joerg Roedel, Jim Mattson, Wanpeng Li,
	Vitaly Kuznetsov, Sean Christopherson, David Matlack

This series avoids kvm_vcpu_gfn_to_memslot() calls during page fault
handling by passing around the memslot in struct kvm_page_fault. This
idea came from Ben Gardon who authored an similar series in Google's
kernel.

This series is an RFC because kvm_vcpu_gfn_to_memslot() calls are
actually quite cheap after commit fe22ed827c5b ("KVM: Cache the last
used slot index per vCPU") since we always hit the cache. However
profiling shows there is still some time (1-2%) spent in
kvm_vcpu_gfn_to_memslot() and that hot instructions are the memory loads
for kvm->memslots[as_id] and slots->used_slots. This series eliminates
this remaining overhead but at the cost of a bit of code churn.

Design
------

We can avoid the cost of kvm_vcpu_gfn_to_memslot() by looking up the
slot once and passing it around. In fact this is quite easy to do now
that KVM passes around struct kvm_page_fault to most of the page fault
handling code.  We can store the slot there without changing most of the
call sites.

The one exception to this is mmu_set_spte, which does not take a
kvm_page_fault since it is also used during spte prefetching. There are
three memslots lookups under mmu_set_spte:

mmu_set_spte
  rmap_add
    kvm_vcpu_gfn_to_memslot
  rmap_recycle
    kvm_vcpu_gfn_to_memslot
  set_spte
    make_spte
      mmu_try_to_unsync_pages
        kvm_page_track_is_active
          kvm_vcpu_gfn_to_memslot

Avoiding these lookups requires plumbing the slot through all of the
above functions. I explored creating a synthetic kvm_page_fault for
prefetching so that kvm_page_fault could be passed to all of these
functions instead, but that resulted in even more code churn.

Patches
-------

Patches 1-2 are small cleanups related to the series.

Patches 3-4 pass the memslot through kvm_page_fault and use it where
kvm_page_fault is already accessible.

Patches 5-6 plumb the memslot down into the guts of mmu_set_spte to
avoid the remaining memslot lookups.

Performance
-----------

I measured the performance using dirty_log_perf_test and taking the
average "Populate memory time" over 10 runs. To help inform whether or
not different parts of this series is worth the code churn I measured
the performance of pages 1-4 and 1-6 separately.

Test                            | tdp_mmu | kvm/queue | Patches 1-4 | Patches 1-6
------------------------------- | ------- | --------- | ----------- | -----------
./dirty_log_perf_test -v64      | Y       | 5.22s     | 5.20s       | 5.20s
./dirty_log_perf_test -v64 -x64 | Y       | 5.23s     | 5.14s       | 5.14s
./dirty_log_perf_test -v64      | N       | 17.14s    | 16.39s      | 15.36s
./dirty_log_perf_test -v64 -x64 | N       | 17.17s    | 16.60s      | 15.31s

This series provides no performance improvement to the tdp_mmu but
improves the legacy MMU page fault handling by about 10%.

David Matlack (6):
  KVM: x86/mmu: Rename try_async_pf to kvm_faultin_pfn in comment
  KVM: x86/mmu: Fold rmap_recycle into rmap_add
  KVM: x86/mmu: Pass around the memslot in kvm_page_fault
  KVM: x86/mmu: Avoid memslot lookup in page_fault_handle_page_track
  KVM: x86/mmu: Avoid memslot lookup in rmap_add
  KVM: x86/mmu: Avoid memslot lookup in mmu_try_to_unsync_pages

 arch/x86/include/asm/kvm_page_track.h |   4 +-
 arch/x86/kvm/mmu.h                    |   5 +-
 arch/x86/kvm/mmu/mmu.c                | 110 +++++++++-----------------
 arch/x86/kvm/mmu/mmu_internal.h       |   3 +-
 arch/x86/kvm/mmu/page_track.c         |   6 +-
 arch/x86/kvm/mmu/paging_tmpl.h        |  18 ++++-
 arch/x86/kvm/mmu/spte.c               |  11 +--
 arch/x86/kvm/mmu/spte.h               |   9 ++-
 arch/x86/kvm/mmu/tdp_mmu.c            |  12 +--
 9 files changed, 80 insertions(+), 98 deletions(-)

-- 
2.33.0.rc1.237.g0d66db33f3-goog

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