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Subject: Re: [PATCH v9 09/17] x86/split_lock: Handle #AC exception for split
 lock
To:     Xiaoyao Li <xiaoyao.li@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>
Cc:     Sean Christopherson <sean.j.christopherson@intel.com>,
        Fenghua Yu <fenghua.yu@intel.com>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        H Peter Anvin <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Radim Krcmar <rkrcmar@redhat.com>,
        Ashok Raj <ashok.raj@intel.com>,
        Tony Luck <tony.luck@intel.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>,
        Ravi V Shankar <ravi.v.shankar@intel.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        x86 <x86@kernel.org>, kvm@vger.kernel.org
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From:   Paolo Bonzini <pbonzini@redhat.com>
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Date:   Wed, 16 Oct 2019 18:38:47 +0200
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On 16/10/19 18:25, Xiaoyao Li wrote:
>>
>>    3 | Y         |     Y       |  N  |   Y     |   x   | Switch
>> MSR_TEST_CTRL on
>>      |           |             |     |         |       | enter/exit,
>> plus:
>>      |           |             |     |         |       | A) #AC
>> forwarded to guest.
>>      |           |             |     |         |       | B) SIGBUS or
>> KVM exit code
>>
> 
> I just want to get confirmed that in (3), we should split into 2 case:
> 
> a) if host has it enabled, still apply the constraint that guest is
> forcibly enabled? so we don't switch MSR_TEST_CTL.
> 
> b) if host has it disabled, we can switch MSR_TEST_CTL on enter/exit.

That's doable, yes.

Paolo