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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>,
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        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 13:26:35 +0200
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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:

> 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.

Paolo