On Wed, Apr 15, 2020 at 02:43:18PM -0700, Sean Christopherson wrote: > On Wed, Apr 15, 2020 at 11:22:11PM +0200, Thomas Gleixner wrote: > > Sean Christopherson <sean.j.christopherson@xxxxxxxxx> writes: > > > I don't see any way to avoid having KVM differentiate between sld_warn and > > > sld_fatal. Even if KVM is able to virtualize SLD in sld_fatal mode, e.g. > > > by telling the guest it must not try to disable SLD, KVM would still need > > > to know the kernel is sld_fatal so that it can forward that information to > > > the guest. > > > > Huch? There is absolutely zero code like that. The only place where > > sld_state is used is: > > > > + static inline void vmx_update_sld(struct kvm_vcpu *vcpu, bool on) > > + { > > + if (sld_state == sld_warn && guest_cpu_has_feature_sld(vcpu) && > > + on == test_thread_flag(TIF_SLD)) { > > + sld_update_msr(on); > > + update_thread_flag(TIF_SLD, !on); > > + } > > > > You might have some faint memories from the previous trainwrecks :) > > Yeah, I was thinking SLD was only being exposed if the host is sld_warn. > I'll work with Xiaoyao to figure out a cleaner interface for this code. ... > > So we can go with the proposed mode of allowing the write but not > > propagating it. If the resulting split lock #AC originates from CPL != 3 > > then the guest will be killed with SIGBUS. If it originates from CPL == > > 3 and the guest has user #AC disabled then it will be killed as well. > > An idea that's been floated around to avoid killing the guest on a CPL==3 > split-lock #AC is to add a STICKY bit to MSR_TEST_CTRL that KVM can > virtualize to tell the guest that attempting to disable SLD is futile, > e.g. so that the guest can kill its misbehaving userspace apps instead of > trying to disable SLD and getting killed by the host. Circling back to this. KVM needs access to sld_state in one form or another if we want to add a KVM hint when the host is in fatal mode. Three options I've come up with: 1. Bite the bullet and export sld_state. 2. Add an is_split_fatal_wrapper(). Ugly since it needs to be non-inline to avoid triggering (1). 3. Add a synthetic feature flag, e.g. X86_FEATURE_SLD_FATAL, and drop sld_state altogether. I like (3) because it requires the least amount of code when all is said and done, doesn't require more exports, and as a bonus it'd probably be nice for userspace to see sld_fatal in /proc/cpuinfo. Thoughts?