Re: [PATCH] x86/split_lock: Don't write MSR_TEST_CTRL on CPUs that aren't whitelisted

[Sean Christopherson]

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.