On 23 April 2024 18:59:21 BST, "Chen, Zide" <zide.chen@xxxxxxxxx> wrote: > > >On 4/23/2024 12:49 AM, David Woodhouse wrote: >>> If I restored the KVM_REQ_GLOBAL_CLOCK_UPDATE request from >>> kvm_arch_vcpu_load(), the selftest works for me, and I ran the test for >>> 1000+ iterations, w/ or w/o TSC scaling, the TEST_ASSERT(delta_corrected >>> <= ±1) never got hit. This is awesome! >>> >>> However, without KVM_REQ_GLOBAL_CLOCK_UPDATE, it still fails on creating >>> a VM. Maybe the init sequence sill needs some rework. >> >> That one confuses me. The crash is actually in debugfs, as it's >> registering the per-vm or per-vcpu stats. I can't imagine *how* that's >> occurring. Or see why the availability of TSC scaling would cause it to >> show up for you and not me. Can I have your .config please? >> >> First thought would be that there's some change in the KVM structures >> and you have some stale object files using the old struct, but then I >> realise I forgot to actually *remove* the now-unused >> kvmclock_update_work from x86's struct kvm_arch anyway. >> >> I'll try to reproduce, as I think I want to *know* what's going on >> here, even if I am going to drop that patch as mentioned in >> https://lore.kernel.org/kvm/a6723ac9e0169839cb33e8022a47c2de213866ac.camel@xxxxxxxxxxxxx >> >> Are you able to load that vmlinux in gdb and >> (gdb) list *start_creating+0x80 >> (gdb) list *kvm_create_vm_debugfs+0x28b >> >> Thanks again. > >My apologies, it turns out the KVM_REQ_GLOBAL_CLOCK_UPDATE is not >needed. Today I can't reproduce the issue after removing it. Yesterday >I thought it may miss something related to pfncache. > >To be clear, with the above mentioned change to >kvm_scale_tsc(master_tsc_scaling_ratio), the selftest runs reliably >regardless TSC scaling is enabled or not. Thanks. That version is now in my git tree and I have tested it myself on Skylake. Then I got distracted by reverse-engineering kvm_get_time_scale() so I could actually add some comments to it. I'm still going to have to put the clock updates back though, for the non-masterclock case. While I'm ripping all this up I guess I ought to rename it to "reference clock" too?
