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From:   Andy Lutomirski <luto@amacapital.net>
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Subject: Re: [PATCH v10 6/6] x86/split_lock: Enable split lock detection by kernel parameter
Date:   Thu, 21 Nov 2019 18:57:44 -0800
Message-Id: <863BD058-5DB8-4C87-B799-29CCB5376FE2@amacapital.net>
References: <fee43477-337a-8de3-9788-e8c8d58d0116@intel.com>
Cc:     Sean Christopherson <sean.j.christopherson@intel.com>,
        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>
In-Reply-To: <fee43477-337a-8de3-9788-e8c8d58d0116@intel.com>
To:     Xiaoyao Li <xiaoyao.li@intel.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Nov 21, 2019, at 6:39 PM, Xiaoyao Li <xiaoyao.li@intel.com> wrote:
>=20
> =EF=BB=BFOn 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:
>>>=20
>>> =EF=BB=BFOn 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:
>>>>>=20
>>>>>> On Nov 21, 2019, at 2:29 PM, Luck, Tony <tony.luck@intel.com> wrote:
>>>>>>=20
>>>>>>> It would be really, really nice if we could pass this feature throug=
h to a VM. Can we?
>>>>>>=20
>>>>>> It's hard because the MSR is core scoped rather than thread scoped.  S=
o on an HT
>>>>>> enabled system a pair of logical processors gets enabled/disabled tog=
ether.
>>>>>>=20
>>>>>=20
>>>>> Well that sucks.
>>>>>=20
>>>>> Could we pass it through if the host has no HT?  Debugging is *so* muc=
h
>>>>> easier in a VM.  And HT is a bit dubious these days anyway.
>>>>=20
>>>> I think it's doable to pass it through to KVM. The difficulty is to dis=
able
>>>> 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 KV=
M,
>>>> it's doable to debug split lock in KVM.
>>>>=20
>>>> Sean and Xiaoyao are working on split lock for KVM (in separate patch s=
et).
>>>> They may have insight on how to do this.
>>>=20
>>> Yes, with SMT off KVM could allow the guest to enable split lock #AC, bu=
t
>>> for the initial implementation we'd want to allow it if and only if spli=
t
>>> lock #AC is disabled in the host kernel.  Otherwise we have to pull in t=
he
>>> 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=E2=80=99s the actual issue?  There=E2=80=99s a window around entry a=
nd exit when a split lock in the host might not give #AC, but as long as no u=
ser code is run, this doesn=E2=80=99t seem like a big problem.
> The problem is that guest can trigger split locked memory access just by d=
isabling split lock #AC even when host has it enabled. In this situation, th=
ere is bus lock held on the hardware without #AC triggered, which is conflic=
t with the purpose that host enables split lock #AC

Fair enough. You need some way to get this enabled in guests eventually, tho=
ugh.=