Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Wed, 16 Oct 2019 08:41:16 -0700
From:   Sean Christopherson <sean.j.christopherson@intel.com>
To:     Paolo Bonzini <pbonzini@redhat.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
Subject: Re: [PATCH v9 09/17] x86/split_lock: Handle #AC exception for split
 lock
Message-ID: <20191016154116.GA5866@linux.intel.com>
References: <20190626203637.GC245468@romley-ivt3.sc.intel.com>
 <alpine.DEB.2.21.1906262338220.32342@nanos.tec.linutronix.de>
 <20190925180931.GG31852@linux.intel.com>
 <3ec328dc-2763-9da5-28d6-e28970262c58@redhat.com>
 <alpine.DEB.2.21.1910161142560.2046@nanos.tec.linutronix.de>
 <57f40083-9063-5d41-f06d-fa1ae4c78ec6@redhat.com>
 <alpine.DEB.2.21.1910161244060.2046@nanos.tec.linutronix.de>
 <3a12810b-1196-b70a-aa2e-9fe17dc7341a@redhat.com>
 <b2c42a64-eb42-1f18-f609-42eec3faef18@intel.com>
 <d2fc3cbe-1506-94fc-73a4-8ed55dc9337d@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <d2fc3cbe-1506-94fc-73a4-8ed55dc9337d@redhat.com>
User-Agent: Mutt/1.5.24 (2015-08-30)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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.