I've recently been looking at our entry/exit costs, and profiling figures did show some very low hanging fruits. The most obvious cost is that accessing the GIC HW is slow. As in "deadly slow", specially when GICv2 is involved. So not hammering the HW when there is nothing to write is immediately beneficial, as this is the most common cases (whatever people seem to think, interrupts are a *rare* event). Another easy thing to fix is the way we handle trapped system registers. We do insist on (mostly) sorting them, but we do perform a linear search on trap. We can switch to a binary search for free, and get immediate benefits (the PMU code, being extremely trap-happy, benefits immediately from this). With these in place, I see an improvement of 20 to 30% (depending on the platform) on our world-switch cycle count when running a set of hand-crafted guests that are designed to only perform traps. Methodology: * NULL-hypercall guest: Perform 65536 PSCI_0_2_FN_PSCI_VERSION calls, and then a power-off: __start: mov x19, #(1 << 16) 1: mov x0, #0x84000000 hvc #0 sub x19, x19, #1 cbnz x19, 1b mov x0, #0x84000000 add x0, x0, #9 hvc #0 b . * sysreg trap guest: Perform 2^20 PMSELR_EL0 accesses, and power-off: __start: mov x19, #(1 << 20) 1: mrs x0, PMSELR_EL0 sub x19, x19, #1 cbnz x19, 1b mov x0, #0x84000000 add x0, x0, #9 hvc #0 b . * These guests are profiled using perf and kvmtool: taskset -c 1 perf stat -e cycles:kh lkvm run -c1 --kernel do_sysreg.bin 2>&1 >/dev/null| grep cycles The result is then divided by the number of iterations (2^16 or 2^20). These tests have been run on Seattle, Mustang, and LS2085, and shown significant improvements in all cases. I've only touched the arm64 GIC code, but obviously the 32bit code should use it as well once we've migrated it to C. I've pushed out a branch (kvm-arm64/suck-less) to the usual location. Thanks, M. Marc Zyngier (8): arm64: KVM: Switch the sys_reg search to be a binary search ARM: KVM: Properly sort the invariant table ARM: KVM: Enforce sorting of all CP tables ARM: KVM: Rename struct coproc_reg::is_64 to is_64bit ARM: KVM: Switch the CP reg search to be a binary search KVM: arm/arm64: timer: Add active state caching KVM: arm/arm64: Avoid accessing GICH registers KVM: arm64: Avoid accessing ICH registers arch/arm/kvm/arm.c | 1 + arch/arm/kvm/coproc.c | 74 ++++++----- arch/arm/kvm/coproc.h | 8 +- arch/arm64/kvm/hyp/vgic-v2-sr.c | 71 +++++++--- arch/arm64/kvm/hyp/vgic-v3-sr.c | 288 ++++++++++++++++++++++++---------------- arch/arm64/kvm/sys_regs.c | 40 +++--- include/kvm/arm_arch_timer.h | 5 + include/kvm/arm_vgic.h | 8 +- virt/kvm/arm/arch_timer.c | 31 +++++ virt/kvm/arm/vgic-v3.c | 4 +- 10 files changed, 334 insertions(+), 196 deletions(-) -- 2.1.4 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
