[PATCH 0/3] KVM: Performance and correctness fixes for CLEAR_DIRTY_LOG

[David Matlack <dmatlack@google.com>]

This series reduces the impact of CLEAR_DIRTY_LOG on guest performance
(Patch 3) and fixes 2 minor bugs found along the way (Patches 1 and 2).

We've observed that guest performance can drop while userspace is
issuing CLEAR_DIRTY_LOG ioctls and tracked down the problem to
contention on the mmu_lock in vCPU threads. CLEAR_DIRTY_LOG holds the
write-lock, so this isn't that surprising. We previously explored
converting CLEAR_DIRTY_LOG to hold the read-lock [1], but that has some
negative consequences:

 - Pretty significant code churn is required on x86 and ARM to support
   doing CLEAR under the read-lock. Things get especially hairy on x86
   when considering how to support the Shadow MMU.

 - Holding the read-lock means KVM will have to use atomic
   compare-and-exchange operations during eager splitting and clearing
   dirty bits, which can be quite slow on certain ARM platforms.

This series proposed an alternative (well, complimentary, really)
approach of simply dropping mmu_lock more frequently. I tested this
series out with one of our internal Live Migration tests where the guest
is running MySQL in a 160 vCPU VM (Intel Broadwell host) and it
eliminates the performance drops we were seeing when userspace issues
CLEAR ioctls. Furthermore I don't see any noticeable improvement when I
test with this series plus a prototype patch convert CLEAR to the read
lock on x86. i.e. It seems we can eliminate most of the lock contention
by just dropping the lock more frequently.

Cc: Vipin Sharma <vipinsh@google.com>

[1] https://lore.kernel.org/kvm/20230602160914.4011728-1-vipinsh@google.com/

David Matlack (3):
  KVM: x86/mmu: Fix off-by-1 when splitting huge pages during CLEAR
  KVM: x86/mmu: Check for leaf SPTE when clearing dirty bit in the TDP
    MMU
  KVM: Aggressively drop and reacquire mmu_lock during CLEAR_DIRTY_LOG

 arch/x86/kvm/mmu/mmu.c     | 2 +-
 arch/x86/kvm/mmu/tdp_mmu.c | 7 ++++---
 virt/kvm/kvm_main.c        | 4 ++--
 3 files changed, 7 insertions(+), 6 deletions(-)


base-commit: 2b3f2325e71f09098723727d665e2e8003d455dc
-- 
2.42.0.820.g83a721a137-goog