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Subject: Re: [PATCH v10 6/6] x86/split_lock: Enable split lock detection by
 kernel parameter
To:     Andy Lutomirski <luto@amacapital.net>,
        Sean Christopherson <sean.j.christopherson@intel.com>
Cc:     Fenghua Yu <fenghua.yu@intel.com>,
        "Luck, Tony" <tony.luck@intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        H Peter Anvin <hpa@zytor.com>,
        "Raj, Ashok" <ashok.raj@intel.com>,
        "Shankar, Ravi V" <ravi.v.shankar@intel.com>,
        linux-kernel <linux-kernel@vger.kernel.org>, x86 <x86@kernel.org>
References: <20191122015225.GG16617@linux.intel.com>
 <81CEB9B7-79EA-4B02-A79C-C9113331A28A@amacapital.net>
From:   Xiaoyao Li <xiaoyao.li@intel.com>
Message-ID: <fee43477-337a-8de3-9788-e8c8d58d0116@intel.com>
Date:   Fri, 22 Nov 2019 10:39:29 +0800
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On 11/22/2019 10:21 AM, Andy Lutomirski wrote:
> 
>> On Nov 21, 2019, at 5:52 PM, Sean Christopherson <sean.j.christopherson@intel.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@intel.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.
> 
The problem is that guest can trigger split locked memory access just by 
disabling split lock #AC even when host has it enabled. In this 
situation, there is bus lock held on the hardware without #AC triggered, 
which is conflict with the purpose that host enables split lock #AC