linux-kernel - Re: [PATCH v10 4/6] x86/split_lock: Enumerate split lock detection if the IA32_CORE

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Date:   Thu, 21 Nov 2019 18:13:18 -0800
From:   Andy Lutomirski <luto@...capital.net>
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>,
        Peter Zijlstra <peterz@...radead.org>,
        Tony Luck <tony.luck@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [PATCH v10 4/6] x86/split_lock: Enumerate split lock detection if the IA32_CORE_CAPABILITIES MSR is not supported


> On Nov 21, 2019, at 4:25 PM, Fenghua Yu <fenghua.yu@...el.com> wrote:
> 
> On Thu, Nov 21, 2019 at 02:07:38PM -0800, Andy Lutomirski wrote:
>> 
>> 
>>>> On Nov 20, 2019, at 5:45 PM, Fenghua Yu <fenghua.yu@...el.com> wrote:
>>> 
>>> Architecturally the split lock detection feature is enumerated by
>>> IA32_CORE_CAPABILITIES MSR and future CPU models will indicate presence
>>> of the feature by setting bit 5. But the feature is present in a few
>>> older models where split lock detection is enumerated by the CPU models.
>>> 
>>> Use a "x86_cpu_id" table to list the older CPU models with the feature.
>>> 
>> 
>> This may need to be disabled if the HYPERVISOR bit is set.
> 
> How about just keeping this patch set as basic enabling code and
> keep HYPERVISOR out of scope as of now? KVM folks will have better
> handling of split lock in KVM once this patch set is available in
> the kernel.
> 
> 

You seem to be assuming that certain model CPUs have this feature even if not enumerated. You need to make sure you don’t try to use it in a VM without the hypervisor giving you an indication that it’s available and permitted. My suggestion is to disable model-based enumeration if HYPERVISOR is set.  You should also consider probing the MSR to double check even if you don’t think you have a hypervisor.