Re: [PATCH v8 4/4] kvm: vmx: virtualize split lock detection

[Sean Christopherson <sean.j.christopherson@intel.com>]

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@intel.com> 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?