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

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Message-Id: <81CEB9B7-79EA-4B02-A79C-C9113331A28A@amacapital.net>
Date:   Thu, 21 Nov 2019 18:21:20 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Fenghua Yu <fenghua.yu@...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 Nov 21, 2019, at 5:52 PM, Sean Christopherson <sean.j.christopherson@...el.com> wrote:
> 
> On Thu, Nov 21, 2019 at 03:53:29PM -0800, Fenghua Yu wrote:
>>> 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.
> 
> Yes, with SMT off KVM could allow the guest to enable split lock #AC, but
> for the initial implementation we'd want to allow it if and only if split
> lock #AC is disabled in the host kernel.  Otherwise we have to pull in the
> logic to control whether or not a guest can disable split lock #AC, what
> to do if a split lock #AC happens when it's enabled by the host but
> disabled by the guest, etc...

What’s the actual issue?  There’s a window around entry and exit when a split lock in the host might not give #AC, but as long as no user code is run, this doesn’t seem like a big problem.