Re: [PATCH 2/2] KVM: VMX: Extend VMX's #AC handding

[Xiaoyao Li]

On 2/1/2020 5:33 AM, Andy Lutomirski wrote:
> > On Jan 31, 2020, at 1:04 PM, Sean Christopherson <sean.j.christopherson@xxxxxxxxx> wrote:
> >
> > ïOn Fri, Jan 31, 2020 at 12:57:51PM -0800, Andy Lutomirski wrote:
> > > > > On Jan 31, 2020, at 12:18 PM, Sean Christopherson <sean.j.christopherson@xxxxxxxxx> wrote:
> > > >
> > > > This is essentially what I proposed a while back.  KVM would allow enabling
> > > > split-lock #AC in the guest if and only if SMT is disabled or the enable bit
> > > > is per-thread, *or* the host is in "warn" mode (can live with split-lock #AC
> > > > being randomly disabled/enabled) and userspace has communicated to KVM that
> > > > it is pinning vCPUs.
> > >
> > > How about covering the actual sensible case: host is set to fatal?  In this
> > > mode, the guest gets split lock detection whether it wants it or not. How do
> > > we communicate this to the guest?
> >
> > KVM doesn't advertise split-lock #AC to the guest and returns -EFAULT to the
> > userspace VMM if the guest triggers a split-lock #AC.
> >
> > Effectively the same behavior as any other userspace process, just that KVM
> > explicitly returns -EFAULT instead of the process getting a SIGBUS.
>
>
> Which helps how if the guest is actually SLD-aware?
>
> I suppose we could make the argument that, if an SLD-aware guest gets #AC at CPL0, itâs a bug, but it still seems rather nicer to forward the #AC to the guest instead of summarily killing it.

If KVM does advertise split-lock detection to the guest, then kvm/host 
can know whether a guest is SLD-aware by checking guest's 
MSR_TEST_CTRL.SPLIT_LOCK_DETECT bit.

 - If guest's MSR_TEST_CTRL.SPLIT_LOCK_DETECT is set, it indicates 
guest is SLD-aware so KVM forwards #AC to guest.

 - If not set. It may be a old guest or a malicious guest or a guest 
without SLD support, and we cannot figure it out. So we have to kill the 
guest when host is SLD-fatal, and let guest survive when SLD-WARN for 
old sane buggy guest.

In a word, all the above is on the condition that KVM advertise 
split-lock detection to guest. But this patch doesn't do this. Maybe I 
should add that part in v2.

> ISTM, on an SLD-fatal host with an SLD-aware guest, the host should tell the guest âhey, you may not do split locks â SLD is forced onâ and the guest should somehow acknowledge it so that it sees the architectural behavior instead of something we made up.  Hence my suggestion.