linux-kernel - Re: [PATCH v9 09/17] x86/split_lock: Handle #AC exception for split lock

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Message-ID: <20191016154116.GA5866@linux.intel.com>
Date:   Wed, 16 Oct 2019 08:41:16 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Xiaoyao Li <xiaoyao.li@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Radim Krcmar <rkrcmar@...hat.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        x86 <x86@...nel.org>, kvm@...r.kernel.org
Subject: Re: [PATCH v9 09/17] x86/split_lock: Handle #AC exception for split
 lock

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?

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.