Re: [PATCH v9 11/17] kvm/vmx: Emulate MSR TEST_CTL

[Xiaoyao Li]

On Thu, 2019-06-27 at 14:11 +0200, Thomas Gleixner wrote:
> On Thu, 27 Jun 2019, Xiaoyao Li wrote:
> > On 6/27/2019 3:12 PM, Thomas Gleixner wrote:
> > > The real interesting question is whether the #AC on split lock prevents
> > > the
> > > actual bus lock or not. If it does then the above is fine.
> > > 
> > > If not, then it would be trivial for a malicious guest to set the
> > > SPLIT_LOCK_ENABLE bit and "handle" the exception pro forma, return to the
> > > offending instruction and trigger another one. It lowers the rate, but
> > > that
> > > doesn't make it any better.
> > > 
> > > The SDM is as usual too vague to be useful. Please clarify.
> > > 
> > 
> > This feature is to ensure no bus lock (due to split lock) in hardware, that
> > to
> > say, when bit 29 of TEST_CTL is set, there is no bus lock due to split lock
> > can be acquired.
> 
> So enabling this prevents the bus lock, i.e. the exception is raised before
> that happens.
> 
exactly.

> Please add that information to the changelog as well because that's
> important to know and makes me much more comfortable handing the #AC back
> into the guest when it has it enabled.
> 
Will add it in next version.

Thanks.