Cc'ing Alex, Darren and Bandan. On 30/7/25 14:39, Igor Mammedov wrote:
v2: * Make both read and write pathes BQL-less (Gerd) * Refactor HPET to handle lock-less access correctly when stopping/starting counter in parallel. (Peter Maydell) * Publish kvm-unit-tests HPET bench/torture test [1] to verify HPET lock-less handlingWhen booting WS2025 with following CLI 1) -M q35,hpet=off -cpu host -enable-kvm -smp 240,sockets=4 the guest boots very slow and is sluggish after boot or it's stuck on boot at spinning circle (most of the time). pref shows that VM is experiencing heavy BQL contention on IO path which happens to be ACPI PM timer read access. A variation with HPET enabled moves contention to HPET timer read access. And it only gets worse with increasing number of VCPUs. Series prevents large VM vCPUs contending on BQL due to PM|HPET timer access and lets Windows to move on with boot process. Testing lock-less IO with HPET micro benchmark [1] shows approx 80% better performance than the current BLQ locked path. [chart https://ibb.co/MJY9999 shows much better scaling of lock-less IO compared to BQL one.] In my tests, with CLI WS2025 guest wasn't able to boot within 30min on both hosts * 32 core 2NUMA nodes * 448 cores 8NUMA nodes With ACPI PM timer in BQL-free read mode, guest boots within approx: * 2min * 1min respectively. With HPET enabled boot time shrinks ~2x * 4m13 -> 2m21 * 2m19 -> 1m15 respectively. 1) "[kvm-unit-tests PATCH v4 0/5] x86: add HPET counter tests" https://lore.kernel.org/kvm/[email protected]/T/#t PS: Using hv-time=on cpu option helps a lot (when it works) and lets [1] guest boot fine in ~1-2min. Series doesn't make a significant impact in this case. PS2: Tested series with a bunch of different guests: RHEL-[6..10]x64, WS2012R2, WS2016, WS2022, WS2025 PS3: dropped mention of https://bugzilla.redhat.com/show_bug.cgi?id=1322713 as it's not reproducible with current software stack or even with the same qemu/seabios as reported (kernel versions mentioned in the report were interim ones and no longer available, so I've used nearest released at the time for testing) Igor Mammedov (6): memory: reintroduce BQL-free fine-grained PIO/MMIO acpi: mark PMTIMER as unlocked hpet: switch to fain-grained device locking hpet: move out main counter read into a separate block hpet: make main counter read lock-less kvm: i386: irqchip: take BQL only if there is an interrupt include/system/memory.h | 10 +++++++ hw/acpi/core.c | 1 + hw/timer/hpet.c | 64 +++++++++++++++++++++++++++++++++++------ system/memory.c | 6 ++++ system/physmem.c | 2 +- target/i386/kvm/kvm.c | 58 +++++++++++++++++++++++-------------- 6 files changed, 111 insertions(+), 30 deletions(-)
