[PATCH v2 00/28] Allow parallel MMU operations with TDP MMU
From: Ben Gardon
Date: Tue Feb 02 2021 - 14:00:07 EST
The TDP MMU was implemented to simplify and improve the performance of
KVM's memory management on modern hardware with TDP (EPT / NPT). To build
on the existing performance improvements of the TDP MMU, add the ability
to handle vCPU page faults, enabling and disabling dirty logging, and
removing mappings, in parallel. In the current implementation,
vCPU page faults (actually EPT/NPT violations/misconfigurations) are the
largest source of MMU lock contention on VMs with many vCPUs. This
contention, and the resulting page fault latency, can soft-lock guests
and degrade performance. Handling page faults in parallel is especially
useful when booting VMs, enabling dirty logging, and handling demand
paging. In all these cases vCPUs are constantly incurring page faults on
each new page accessed.
Broadly, the following changes were required to allow parallel page
faults (and other MMU operations):
-- Contention detection and yielding added to rwlocks to bring them up to
feature parity with spin locks, at least as far as the use of the MMU
lock is concerned.
-- TDP MMU page table memory is protected with RCU and freed in RCU
callbacks to allow multiple threads to operate on that memory
concurrently.
-- The MMU lock was changed to an rwlock on x86. This allows the page
fault handlers to acquire the MMU lock in read mode and handle page
faults in parallel, and other operations to maintain exclusive use of
the lock by acquiring it in write mode.
-- An additional lock is added to protect some data structures needed by
the page fault handlers, for relatively infrequent operations.
-- The page fault handler is modified to use atomic cmpxchgs to set SPTEs
and some page fault handler operations are modified slightly to work
concurrently with other threads.
This series also contains a few bug fixes and optimizations, related to
the above, but not strictly part of enabling parallel page fault handling.
Correctness testing:
The following tests were performed with an SMP kernel and DBX kernel on an
Intel Skylake machine. The tests were run both with and without the TDP
MMU enabled.
-- This series introduces no new failures in kvm-unit-tests
SMP + no TDP MMU no new failures
SMP + TDP MMU no new failures
DBX + no TDP MMU no new failures
DBX + TDP MMU no new failures
-- All KVM selftests behave as expected
SMP + no TDP MMU all pass except ./x86_64/vmx_preemption_timer_test
SMP + TDP MMU all pass except ./x86_64/vmx_preemption_timer_test
(./x86_64/vmx_preemption_timer_test also fails without this patch set,
both with the TDP MMU on and off.)
DBX + no TDP MMU all pass
DBX + TDP MMU all pass
-- A VM can be booted running a Debian 9 and all memory accessed
SMP + no TDP MMU works
SMP + TDP MMU works
DBX + no TDP MMU works
DBX + TDP MMU works
This series can be viewed in Gerrit at:
https://linux-review.googlesource.com/c/linux/kernel/git/torvalds/linux/+/7172
Changelog v1 -> v2:
- Removed the MMU lock union + using a spinlock when the TDP MMU is disabled
- Merged RCU commits
- Extended additional MMU operations to operate in parallel
- Ammended dirty log perf test to cover newly parallelized code paths
- Misc refactorings (see changelogs for individual commits)
- Big thanks to Sean and Paolo for their thorough review of v1
Ben Gardon (28):
KVM: x86/mmu: change TDP MMU yield function returns to match
cond_resched
KVM: x86/mmu: Add comment on __tdp_mmu_set_spte
KVM: x86/mmu: Add lockdep when setting a TDP MMU SPTE
KVM: x86/mmu: Don't redundantly clear TDP MMU pt memory
KVM: x86/mmu: Factor out handling of removed page tables
locking/rwlocks: Add contention detection for rwlocks
sched: Add needbreak for rwlocks
sched: Add cond_resched_rwlock
KVM: x86/mmu: Fix braces in kvm_recover_nx_lpages
KVM: x86/mmu: Fix TDP MMU zap collapsible SPTEs
KVM: x86/mmu: Merge flush and non-flush tdp_mmu_iter_cond_resched
KVM: x86/mmu: Rename goal_gfn to next_last_level_gfn
KVM: x86/mmu: Ensure forward progress when yielding in TDP MMU iter
KVM: x86/mmu: Yield in TDU MMU iter even if no SPTES changed
KVM: x86/mmu: Skip no-op changes in TDP MMU functions
KVM: x86/mmu: Clear dirtied pages mask bit before early break
KVM: x86/mmu: Protect TDP MMU page table memory with RCU
KVM: x86/mmu: Use an rwlock for the x86 MMU
KVM: x86/mmu: Factor out functions to add/remove TDP MMU pages
KVM: x86/mmu: Use atomic ops to set SPTEs in TDP MMU map
KVM: x86/mmu: Flush TLBs after zap in TDP MMU PF handler
KVM: x86/mmu: Mark SPTEs in disconnected pages as removed
KVM: x86/mmu: Allow parallel page faults for the TDP MMU
KVM: x86/mmu: Allow zap gfn range to operate under the mmu read lock
KVM: x86/mmu: Allow zapping collapsible SPTEs to use MMU read lock
KVM: x86/mmu: Allow enabling / disabling dirty logging under MMU read
lock
KVM: selftests: Add backing src parameter to dirty_log_perf_test
KVM: selftests: Disable dirty logging with vCPUs running
arch/x86/include/asm/kvm_host.h | 15 +
arch/x86/kvm/mmu/mmu.c | 120 +--
arch/x86/kvm/mmu/mmu_internal.h | 9 +-
arch/x86/kvm/mmu/page_track.c | 8 +-
arch/x86/kvm/mmu/paging_tmpl.h | 8 +-
arch/x86/kvm/mmu/spte.h | 21 +-
arch/x86/kvm/mmu/tdp_iter.c | 46 +-
arch/x86/kvm/mmu/tdp_iter.h | 21 +-
arch/x86/kvm/mmu/tdp_mmu.c | 741 ++++++++++++++----
arch/x86/kvm/mmu/tdp_mmu.h | 5 +-
arch/x86/kvm/x86.c | 4 +-
include/asm-generic/qrwlock.h | 24 +-
include/linux/kvm_host.h | 5 +
include/linux/rwlock.h | 7 +
include/linux/sched.h | 29 +
kernel/sched/core.c | 40 +
.../selftests/kvm/demand_paging_test.c | 3 +-
.../selftests/kvm/dirty_log_perf_test.c | 25 +-
.../testing/selftests/kvm/include/kvm_util.h | 6 -
.../selftests/kvm/include/perf_test_util.h | 3 +-
.../testing/selftests/kvm/include/test_util.h | 14 +
.../selftests/kvm/lib/perf_test_util.c | 6 +-
tools/testing/selftests/kvm/lib/test_util.c | 29 +
virt/kvm/dirty_ring.c | 10 +
virt/kvm/kvm_main.c | 46 +-
25 files changed, 963 insertions(+), 282 deletions(-)
--
2.30.0.365.g02bc693789-goog