Though not explicitly stated, for all intents and purposes the fast
invalidate mechanism was added to speed up the scenario where removing
a memslot, e.g. when accessing PCI ROM, caused KVM to flush all shadow
entries[1].
The other use cases of "flush everything" are VM teardown and handling
MMIO generation overflow, neither of which is a performance critial
path (see "Other Uses" below).
For the memslot case, zapping all shadow entries is overkill, i.e. KVM
only needs to zap the entries associated with the memslot, but KVM has
historically used a big hammer because if all you have is a hammer,
everything looks like a nail.
Rather than zap all pages when removing a memslot, zap only the shadow
entries associated with said memslot. I see a performance improvement
of ~5% when zapping only the pages for the deleted memslot when using a
slightly modified version of the original benchmark[2][3][4] (I don't
have easy access to a system with hundreds of gigs of memory).
$ cat shell.sh
#!/bin/sh
echo 3 > /proc/sys/vm/drop_caches
./mmtest -c 8 -m 2000 -e ./rom.sh
I.e. running 8 threads and 2gb of memory per thread, time in milliseconds:
Before: 89.117
After: 84.768
With the memslot use case gone, maintaining the fast invalidate path
adds a moderate amount of complexity but provides little to no value.
Furhtermore, its existence may give the impression that it's "ok" to zap
all shadow pages. Remove the fast invalidate mechanism to simplify the
code and to discourage future code from zapping all pages as using such
a big hammer should be a last resort.
History:
Flushing of shadow pages when removing a memslot was originally added
by commit 34d4cb8fca1f ("KVM: MMU: nuke shadowed pgtable pages and ptes
on memslot destruction"), and obviously emphasized functionality over
performance. Commit 2df72e9bc4c5 ("KVM: split kvm_arch_flush_shadow")
added a path to allow flushing only the removed slot's shadow pages,
but x86 just redirected to the "zap all" flow.
Eventually, it became evident that zapping everything is slow, and so
x86 developed a more efficient hammer in the form of the fast invalidate
mechanism.
Other Uses:
When a VM is being destroyed, either there are no active vcpus, i.e.
there's no lock contention, or the VM has ungracefully terminated, in
which case we want to reclaim its pages as quickly as possible, i.e.
not release the MMU lock if there are still CPUs executing in the VM.
The MMIO generation scenario is almost literally a one-in-a-million
occurrence, i.e. is not a performance sensitive scenario.
It's worth noting that prior to the "fast invalidate" series being
applied, there was actually another use case of kvm_mmu_zap_all() in
emulator_fix_hypercall(). That may have contributed to improving the
performance of "zap all" instead of avoiding it altogether, but that
usage was removed by the series itself in commit 758ccc89b83c ("KVM:
x86: drop calling kvm_mmu_zap_all in emulator_fix_hypercall").
[1] https://lkml.kernel.org/r/1369960590-14138-1-git-send-email-xiaoguangrong@xxxxxxxxxxxxxxxxxx
[2] https://lkml.kernel.org/r/1368706673-8530-1-git-send-email-xiaoguangrong@xxxxxxxxxxxxxxxxxx
[3] http://lkml.iu.edu/hypermail/linux/kernel/1305.2/00277.html
[4] http://lkml.iu.edu/hypermail/linux/kernel/1305.2/00277/mmtest.tar.bz2
Sean Christopherson (14):
KVM: x86/mmu: Move slot_level_*() helper functions up a few lines
KVM: x86/mmu: Split remote_flush+zap case out of
kvm_mmu_flush_or_zap()
KVM: x86/mmu: Zap only the relevant pages when removing a memslot
Revert "KVM: MMU: document fast invalidate all pages"
Revert "KVM: MMU: drop kvm_mmu_zap_mmio_sptes"
KVM: x86/mmu: Remove is_obsolete() call
Revert "KVM: MMU: reclaim the zapped-obsolete page first"
Revert "KVM: MMU: collapse TLB flushes when zap all pages"
Revert "KVM: MMU: zap pages in batch"
Revert "KVM: MMU: add tracepoint for kvm_mmu_invalidate_all_pages"
Revert "KVM: MMU: show mmu_valid_gen in shadow page related
tracepoints"
Revert "KVM: x86: use the fast way to invalidate all pages"
Revert "KVM: MMU: fast invalidate all pages"
KVM: x86/mmu: Consolidate kvm_mmu_zap_all() and
kvm_mmu_zap_mmio_sptes()
Documentation/virtual/kvm/mmu.txt | 28 +--
arch/x86/include/asm/kvm_host.h | 7 +-
arch/x86/kvm/mmu.c | 325 ++++++++++++------------------
arch/x86/kvm/mmu.h | 1 -
arch/x86/kvm/mmutrace.h | 42 +---
arch/x86/kvm/x86.c | 3 +-
6 files changed, 145 insertions(+), 261 deletions(-)
--
2.19.2
