linux-kernel - Re: [PATCH v5 3/9] x86/split_lock: Re-define the kernel param option for split_lock

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Message-ID: <20200324180207.GD5998@linux.intel.com>
Date:   Tue, 24 Mar 2020 11:02:07 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Xiaoyao Li <xiaoyao.li@...el.com>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>, hpa@...or.com,
        Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH v5 3/9] x86/split_lock: Re-define the kernel param option
 for split_lock_detect

On Tue, Mar 24, 2020 at 11:40:18AM +0100, Thomas Gleixner wrote:
> Xiaoyao Li <xiaoyao.li@...el.com> writes:
> > On 3/24/2020 1:10 AM, Thomas Gleixner wrote:
> >> Xiaoyao Li <xiaoyao.li@...el.com> writes:
> >> 
> >>> Change sld_off to sld_disable, which means disabling feature split lock
> >>> detection and it cannot be used in kernel nor can kvm expose it guest.
> >>> Of course, the X86_FEATURE_SPLIT_LOCK_DETECT is not set.
> >>>
> >>> Add a new optioin sld_kvm_only, which means kernel turns split lock
> >>> detection off, but kvm can expose it to guest.
> >> 
> >> What's the point of this? If the host is not clean, then you better fix
> >> the host first before trying to expose it to guests.
> >
> > It's not about whether or not host is clean. It's for the cases that 
> > users just don't want it enabled on host, to not break the applications 
> > or drivers that do have split lock issue.
> 
> It's very much about whether the host is split lock clean.
> 
> If your host kernel is not, then this wants to be fixed first. If your
> host application is broken, then either fix it or use "warn".

The "kvm only" option was my suggestion.  The thought was to provide a way
for users to leverage KVM to debug/test kernels without having to have a
known good kernel and/or to minimize the risk of crashing their physical
system.  E.g. debug a misbehaving driver by assigning its associated device
to a guest.