On 13/08/21 22:34, David Matlack wrote:
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(-)
Queued patches 1-3, thanks. For the others, see the reply to patch 6.
Paolo
