linux-kernel - Re: [PATCH v4 00/17] x86/split_lock: Enable #AC exception for split locked accesses

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Message-ID: <20190304215505.GC30350@char.us.oracle.com>
Date:   Mon, 4 Mar 2019 16:55:05 -0500
From:   Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:     Fenghua Yu <fenghua.yu@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        Xiaoyao Li <xiaoyao.li@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        x86 <x86@...nel.org>, kvm@...r.kernel.org
Subject: Re: [PATCH v4 00/17] x86/split_lock: Enable #AC exception for split
 locked accesses

> This capability may also find usage in cloud. A user process with split
> lock running in one guest can block other cores from accessing shared
> memory during its split locked memory access. That may cause overall
> system performance degradation.

"shared memory" ? As in memory shared between two guests?

> 
> Split lock may open a security hole where malicious user code may slow
> down overall system by executing instructions with split lock.

How much slowdown are we talking?