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 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 (8): 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 KVM: arm/arm64: Detangle kvm_mmu.h from kvm_hyp.h arch/arm/include/asm/kvm_hyp.h | 3 +- arch/arm/include/asm/kvm_mmu.h | 61 +++++++++++++++++++++++++++----- 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 | 26 ++++++++++++++ arch/arm64/include/asm/cacheflush.h | 8 +++++ arch/arm64/include/asm/kvm_hyp.h | 1 - arch/arm64/include/asm/kvm_mmu.h | 33 ++++++++++++++---- 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 | 26 ++++++++------ virt/kvm/arm/hyp/vgic-v2-sr.c | 1 + virt/kvm/arm/mmu.c | 64 +++++++++++++++++++++++++++------- 17 files changed, 195 insertions(+), 43 deletions(-) -- 2.14.1
