[PATCH v3 0/9] arm/arm64: KVM: limit icache invalidation to prefetch aborts

Marc Zyngier
2017-10-23T16:11:13Z

[now with the patches correctly following...]

It was recently reported that on a VM restore, we seem to spend a
disproportionate amount of time invalidation the icache. This is
partially due to some HW behaviour, but also because we're being a bit
dumb and are invalidating the icache for every page we map at S2, even
if that on a data access.

The slightly better way of doing this is to mark the pages XN at S2,
and wait for the the guest to execute something in that page, at which
point we perform the invalidation. As it is likely that there is a lot
less instruction than data, we win (or so we hope).

We also take this opportunity to drop the extra dcache clean to the
PoU which is pretty useless, as we already clean all the way to the
PoC...

Running a bare metal test that touches 1GB of memory (using a 4kB
stride) leads to the following results on Seattle:

4.13:
do_fault_read.bin: 0.565885992 seconds time elapsed
do_fault_write.bin: 0.738296337 seconds time elapsed
do_fault_read_write.bin: 1.241812231 seconds time elapsed

4.14-rc3+patches:
do_fault_read.bin: 0.244961803 seconds time elapsed
do_fault_write.bin: 0.422740092 seconds time elapsed
do_fault_read_write.bin: 0.643402470 seconds time elapsed

We're almost halving the time of something that more or less looks
like a restore operation. Some larger systems will show much bigger
benefits as they become less impacted by the icache invalidation
(which is broadcast in the inner shareable domain). I've tried to
measure the impact on a VM boot in order to assess the impact of
taking an extra permission fault, but found that any difference was
simply noise.

I've also given it a test run on both Cubietruck and Jetson-TK1.

Tests are archived here:
https://git.kernel.org/pub/scm/linux/kernel/git/maz/kvm-ws-tests.git/

I'd value some additional test results on HW I don't have access to.

* From v2:
- Brought back the "detangling" patch that allows 32bit ARM to still
compile...
- Let arm64 icache invalidation deal with userspace addresses

* From v1:
- Some function renaming (coherent->clean/invalidate)
- Made the arm64 icache invalidation a macro that's now used in
two places
- Fixed BTB flushing on 32bit
- Added stage2_is_exec as a predicate for XN being absent from the
entry
- Dropped patch #10 which was both useless and broken, and patch #9
that thus became useless
- Tried to measure the impact on kernel boot time and failed to see
any difference

Marc Zyngier (9):
KVM: arm/arm64: Detangle kvm_mmu.h from kvm_hyp.h
KVM: arm/arm64: Split dcache/icache flushing
arm64: KVM: Add invalidate_icache_range helper
arm: KVM: Add optimized PIPT icache flushing
arm64: KVM: PTE/PMD S2 XN bit definition
KVM: arm/arm64: Limit icache invalidation to prefetch aborts
KVM: arm/arm64: Only clean the dcache on translation fault
KVM: arm/arm64: Preserve Exec permission across R/W permission faults
KVM: arm/arm64: Drop vcpu parameter from guest cache maintenance
operartions

arch/arm/include/asm/kvm_hyp.h | 3 +-
arch/arm/include/asm/kvm_mmu.h | 99 ++++++++++++++++++++++++++++------
arch/arm/include/asm/pgtable.h | 4 +-
arch/arm/kvm/hyp/switch.c | 1 +
arch/arm/kvm/hyp/tlb.c | 1 +
arch/arm64/include/asm/assembler.h | 21 ++++++++
arch/arm64/include/asm/cacheflush.h | 7 +++
arch/arm64/include/asm/kvm_hyp.h | 1 -
arch/arm64/include/asm/kvm_mmu.h | 36 +++++++++++--
arch/arm64/include/asm/pgtable-hwdef.h | 2 +
arch/arm64/include/asm/pgtable-prot.h | 4 +-
arch/arm64/kvm/hyp/debug-sr.c | 1 +
arch/arm64/kvm/hyp/switch.c | 1 +
arch/arm64/kvm/hyp/tlb.c | 1 +
arch/arm64/mm/cache.S | 32 +++++++----
virt/kvm/arm/hyp/vgic-v2-sr.c | 1 +
virt/kvm/arm/mmu.c | 64 +++++++++++++++++++---
17 files changed, 236 insertions(+), 43 deletions(-)

--
2.11.0