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From:   Andy Lutomirski <luto@amacapital.net>
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Subject: Re: [PATCH 3/3] KVM: VMX: Extend VMX's #AC interceptor to handle split lock #AC in guest
Date:   Thu, 2 Apr 2020 13:36:08 -0700
Message-Id: <D6B8E21D-6DB2-4DF8-8B73-12DD36476F55@amacapital.net>
References: <87h7y1mz2s.fsf@nanos.tec.linutronix.de>
Cc:     Sean Christopherson <sean.j.christopherson@intel.com>,
        x86@kernel.org, "Kenneth R . Crudup" <kenny@panix.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Fenghua Yu <fenghua.yu@intel.com>,
        Xiaoyao Li <xiaoyao.li@intel.com>,
        Nadav Amit <namit@vmware.com>,
        Thomas Hellstrom <thellstrom@vmware.com>,
        Tony Luck <tony.luck@intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Jessica Yu <jeyu@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org
In-Reply-To: <87h7y1mz2s.fsf@nanos.tec.linutronix.de>
To:     Thomas Gleixner <tglx@linutronix.de>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Apr 2, 2020, at 1:07 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
>=20

>=20
>=20
> TBH, the more I learn about this, the more I tend to just give up on
> this whole split lock stuff in its current form and wait until HW folks
> provide something which is actually usable:
>=20
>   - Per thread
>   - Properly distinguishable from a regular #AC via error code

Why the latter?  I would argue that #AC from CPL3 with EFLAGS.AC set is almo=
st by construction not a split lock. In particular, if you meet these condit=
ions, how exactly can you do a split lock without simultaneously triggering a=
n alignment check?  (Maybe CMPXCHG16B?

>=20
> OTOH, that means I won't be able to use it before retirement. Oh well.
>=20
> Thanks,
>=20
>        tglx