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Subject: Re: [PATCH v9 09/17] x86/split_lock: Handle #AC exception for split
 lock
To:     Paolo Bonzini <pbonzini@redhat.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:   Xiaoyao Li <xiaoyao.li@intel.com>
Message-ID: <8808c9ac-0906-5eec-a31f-27cbec778f9c@intel.com>
Date:   Wed, 16 Oct 2019 21:13:04 +0800
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On 10/16/2019 7:26 PM, Paolo Bonzini wrote:
> On 16/10/19 13:23, Xiaoyao Li wrote:
>> KVM always traps #AC, and only advertises split-lock detection to guest
>> when the global variable split_lock_detection_enabled in host is true.
>>
>> - If guest enables #AC (CPL3 alignment check or split-lock detection
>> enabled), injecting #AC back into guest since it's supposed capable of
>> handling it.
>> - If guest doesn't enable #AC, KVM reports #AC to userspace (like other
>> unexpected exceptions), and we can print a hint in kernel, or let
>> userspace (e.g., QEMU) tell the user guest is killed because there is a
>> split-lock in guest.
>>
>> In this way, malicious guests always get killed by userspace and old
>> sane guests cannot survive as well if it causes split-lock. If we do
>> want old sane guests work we have to disable the split-lock detection
>> (through booting parameter or debugfs) in the host just the same as we
>> want to run an old and split-lock generating userspace binary.
> 
> Old guests are prevalent enough that enabling split-lock detection by
> default would be a big usability issue.  And even ignoring that, you
> would get the issue you describe below:

Right, whether enabling split-lock detection is made by the 
administrator. The administrator is supposed to know the consequence of 
enabling it. Enabling it means don't want any split-lock happens in 
userspace, of course VMM softwares are under control.

>> But there is an issue that we advertise split-lock detection to guest
>> based on the value of split_lock_detection_enabled to be true in host,
>> which can be turned into false dynamically when split-lock happens in
>> host kernel.
> 
> ... which means that supposedly safe guests become unsafe, and that is bad.
> 
>> This causes guest's capability changes at run time and I
>> don't if there is a better way to inform guest? Maybe we need a pv
>> interface?
> 
> Even a PV interface would not change the basic fact that a supposedly
> safe configuration becomes unsafe.

I don't catch you about the unsafe?

If host disables split-lock detection dynamically, then the 
MST_TEST_CTL.split_lock is clear in the hardware and we can use the PV 
interface to notify the guest so that guest knows it loses the 
capability of split-lock detection. In this case, I think safety is 
meaningless for both host and guest.

> Paolo
>