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?
