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Date:   Wed, 16 Oct 2019 10:42:00 -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: <20191016174200.GF5866@linux.intel.com>
References: <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>
 <20191016154116.GA5866@linux.intel.com>
 <d235ed9a-314c-705c-691f-b31f2f8fa4e8@redhat.com>
 <20191016162337.GC5866@linux.intel.com>
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On Wed, Oct 16, 2019 at 09:23:37AM -0700, Sean Christopherson wrote:
> On Wed, Oct 16, 2019 at 05:43:53PM +0200, Paolo Bonzini wrote:
> > 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.
> 
> Ah, didn't realize that.  I'm surprised emulating all locks with cmpxchg
> doesn't cause problems (or am I misreading the code?).  Assuming I'm
> reading the code correctly, the #AC path could kick all other vCPUS on
> emulation failure and then retry emulation to "guarantee" success.  Though
> that's starting to build quite the house of cards.

Ugh, doesn't the existing emulation behavior create another KVM issue?
KVM uses a locked cmpxchg in emulator_cmpxchg_emulated() and the address
is guest controlled, e.g. a guest could coerce the host into disabling
split-lock detection via the host's #AC handler by triggering emulation
and inducing an #AC in the emulator.

> > 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?
> 
> Tweak MSR_TEST_CTRL, with logic to handle the scenario where split-lock
> detection is globally disable during emulation (so KVM doesn't
> inadvertantly re-enable it).
> 
> There isn't much for the sibling to notice.  The kernel would temporarily
> allow split-locks on the sibling, but that's a performance issue and isn't
> directly fatal.  A missed #AC in the host kernel would only delay the
> inevitable global disabling of split-lock.  A missed #AC in userspace would
> again just delay the inevitable SIGBUS.