On Sun, Jun 07, 2020 at 10:06:46AM -0700, Sean Christopherson wrote: > On Sat, Jun 06, 2020 at 10:51:06AM +0800, Xiaoyao Li wrote: > > On 6/6/2020 3:26 AM, Sean Christopherson wrote: > > >Choo! Choo! All aboard the Split Lock Express, with direct service to > > >Wreckage! > > > > > >Skip split_lock_verify_msr() if the CPU isn't whitelisted as a possible > > >SLD-enabled CPU model to avoid writing MSR_TEST_CTRL. MSR_TEST_CTRL > > >exists, and is writable, on many generations of CPUs. Writing the MSR, > > >even with '0', can result in bizarre, undocumented behavior. > > > > > >This fixes a crash on Haswell when resuming from suspend with a live KVM > > >guest. Because APs use the standard SMP boot flow for resume, they will > > >go through split_lock_init() and the subsequent RDMSR/WRMSR sequence, > > >which runs even when sld_state==sld_off to ensure SLD is disabled. On > > >Haswell (at least, my Haswell), writing MSR_TEST_CTRL with '0' will > > >succeed and _may_ take the SMT _sibling_ out of VMX root mode. > > > > > >When KVM has an active guest, KVM performs VMXON as part of CPU onlining > > >(see kvm_starting_cpu()). Because SMP boot is serialized, the resulting > > >flow is effectively: > > > > > > on_each_ap_cpu() { > > > WRMSR(MSR_TEST_CTRL, 0) > > > VMXON > > > } > > > > > >As a result, the WRMSR can disable VMX on a different CPU that has > > >already done VMXON. This ultimately results in a #UD on VMPTRLD when > > >KVM regains control and attempt run its vCPUs. > > > > > >The above voodoo was confirmed by reworking KVM's VMXON flow to write > > >MSR_TEST_CTRL prior to VMXON, and to serialize the sequence as above. > > >Further verification of the insanity was done by redoing VMXON on all > > >APs after the initial WRMSR->VMXON sequence. The additional VMXON, > > >which should VM-Fail, occasionally succeeded, and also eliminated the > > >unexpected #UD on VMPTRLD. > > > > > >The damage done by writing MSR_TEST_CTRL doesn't appear to be limited > > >to VMX, e.g. after suspend with an active KVM guest, subsequent reboots > > >almost always hang (even when fudging VMXON), a #UD on a random Jcc was > > >observed, suspend/resume stability is qualitatively poor, and so on and > > >so forth. > > > > > > > I'm wondering if all those side-effects of MSR_TEST_CTRL exist on CPUs have > > SLD feature, have you ever tested on a SLD capable CPU? > > No, I'll poke at it on ICX tomorrow. Tested on ICX with SLD both enabled and disabled by writing MSR_TEST_CTRL on every logical CPU during vCPU creation, no weirdness observed.