linux-kernel - Re: [RFD] x86/split_lock: Request to Intel

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Message-ID: <alpine.DEB.2.21.1910181100000.1869@nanos.tec.linutronix.de>
Date:   Fri, 18 Oct 2019 11:02:37 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Xiaoyao Li <xiaoyao.li@...el.com>
cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Radim Krcmar <rkrcmar@...hat.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        x86 <x86@...nel.org>, kvm@...r.kernel.org
Subject: Re: [RFD] x86/split_lock: Request to Intel

On Fri, 18 Oct 2019, Xiaoyao Li wrote:
> On 10/17/2019 8:29 PM, Thomas Gleixner wrote:
> > The more I look at this trainwreck, the less interested I am in merging any
> > of this at all.
> > 
> > The fact that it took Intel more than a year to figure out that the MSR is
> > per core and not per thread is yet another proof that this industry just
> > works by pure chance.
> > 
> 
> Whether it's per-core or per-thread doesn't affect much how we implement for
> host/native.

How useful.

> And also, no matter it's per-core or per-thread, we always can do something in
> VIRT.

It matters a lot. If it would be per thread then we would not have this
discussion at all.

> Maybe what matters is below.
> 
> > Seriously, this makes only sense when it's by default enabled and not
> > rendered useless by VIRT. Otherwise we never get any reports and none of
> > the issues are going to be fixed.
> > 
> 
> For VIRT, it doesn't want old guest to be killed due to #AC. But for native,
> it doesn't want VIRT to disable the #AC detection
> 
> I think it's just about the default behavior that whether to disable the
> host's #AC detection or kill the guest (SIGBUS or something else) once there
> is an split-lock #AC in guest.
> 
> So we can provide CONFIG option to set the default behavior and module
> parameter to let KVM set/change the default behavior.

Care to read through the whole discussion and figure out WHY it's not that
simple?

Thanks,

	tglx