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From:   Andy Lutomirski <luto@amacapital.net>
Mime-Version: 1.0 (1.0)
Subject: Re: [PATCH v10 6/6] x86/split_lock: Enable split lock detection by kernel parameter
Date:   Thu, 21 Nov 2019 18:21:20 -0800
Message-Id: <81CEB9B7-79EA-4B02-A79C-C9113331A28A@amacapital.net>
References: <20191122015225.GG16617@linux.intel.com>
Cc:     Fenghua Yu <fenghua.yu@intel.com>,
        Xiaoyao Li <xiaoyao.li@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: <20191122015225.GG16617@linux.intel.com>
To:     Sean Christopherson <sean.j.christopherson@intel.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Nov 21, 2019, at 5:52 PM, Sean Christopherson <sean.j.christopherson@in=
tel.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 through t=
o a VM. Can we?
>>>>=20
>>>> 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 toget=
her.
>>>>=20
>>>=20
>>> Well that sucks.
>>>=20
>>> 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.
>>=20
>> I think it's doable to pass it through to KVM. The difficulty is to disab=
le
>> split lock detection in KVM because that will disable split lock on the w=
hole
>> core including threads for the host. Without disabling split lock in KVM,=

>> it's doable to debug split lock in KVM.
>>=20
>> Sean and Xiaoyao are working on split lock for KVM (in separate patch set=
).
>> They may have insight on how to do this.
>=20
> 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=E2=80=99s the actual issue?  There=E2=80=99s a window around entry and e=
xit when a split lock in the host might not give #AC, but as long as no user=
 code is run, this doesn=E2=80=99t seem like a big problem.=