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Subject: Re: [PATCH v9 09/17] x86/split_lock: Handle #AC exception for split
 lock
To:     Sean Christopherson <sean.j.christopherson@intel.com>
Cc:     Xiaoyao Li <xiaoyao.li@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        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 17:43:53 +0200
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On 16/10/19 17:41, Sean Christopherson wrote:
> On Wed, Oct 16, 2019 at 04:08:14PM +0200, Paolo Bonzini wrote:
>> SIGBUS (actually a new KVM_EXIT_INTERNAL_ERROR result from KVM_RUN is
>> better, but that's the idea) is for when you're debugging guests.
>> Global disable (or alternatively, disable SMT) is for production use.
> 
> Alternatively, for guests without split-lock #AC enabled, what if KVM were
> to emulate the faulting instruction with split-lock detection temporarily
> disabled?

Yes we can get fancy, but remember that KVM is not yet supporting
emulation of locked instructions.  Adding it is possible but shouldn't
be in the critical path for the whole feature.

How would you disable split-lock detection temporarily?  Just tweak
MSR_TEST_CTRL for the time of running the one instruction, and cross
fingers that the sibling doesn't notice?

Thanks,

Paolo

> The emulator can presumably handle all such lock instructions, and an
> unhandled instruction would naturally exit to userspace.
> 
> The latency of VM-Enter+VM-Exit should be enough to guard against DoS from
> a malicious guest.  KVM could also artificially rate-limit a guest that is
> generating copious amounts of split-lock #ACs.
>