linux-kernel - Re: [PATCH v10 6/6] x86/split_lock: Enable split lock detection by kernel parameter

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Date:   Thu, 21 Nov 2019 15:53:29 -0800
From:   Fenghua Yu <fenghua.yu@...el.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Christopherson Sean J <sean.j.christopherson@...el.com>,
        Xiaoyao Li <xiaoyao.li@...el.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [PATCH v10 6/6] x86/split_lock: Enable split lock detection by
 kernel parameter

On Thu, Nov 21, 2019 at 03:18:46PM -0800, Andy Lutomirski wrote:
> 
> 
> > On Nov 21, 2019, at 2:29 PM, Luck, Tony <tony.luck@...el.com> wrote:
> > 
> > 
> >> 
> >> It would be really, really nice if we could pass this feature through to a VM. Can we?
> > 
> > It's hard because the MSR is core scoped rather than thread scoped.  So on an HT
> > enabled system a pair of logical processors gets enabled/disabled together.
> > 
> > 
> 
> Well that sucks.
> 
> Could we pass it through if the host has no HT?  Debugging is *so* much easier in a VM.  And HT is a bit dubious these days anyway.

I think it's doable to pass it through to KVM. The difficulty is to disable
split lock detection in KVM because that will disable split lock on
the whole core including threads for the host. Without disabling split lock
in KVM, it's doable to debug split lock in KVM.

Sean and Xiaoyao are working on split lock for KVM (in separate patch set).
They may have insight on how to do this.

Thanks.

-Fenghua