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Subject: Re: [PATCH v9 11/17] kvm/vmx: Emulate MSR TEST_CTL
From:   Xiaoyao Li <xiaoyao.li@intel.com>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Fenghua Yu <fenghua.yu@intel.com>, Ingo Molnar <mingo@redhat.com>,
        Borislav Petkov <bp@alien8.de>, H Peter Anvin <hpa@zytor.com>,
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        Christopherson Sean J <sean.j.christopherson@intel.com>,
        Ashok Raj <ashok.raj@intel.com>,
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        Dan Williams <dan.j.williams@intel.com>,
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        Ravi V Shankar <ravi.v.shankar@intel.com>,
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Date:   Thu, 27 Jun 2019 20:22:28 +0800
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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.