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From:   Andy Lutomirski <luto@amacapital.net>
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Subject: Re: [PATCH 2/2] KVM: VMX: Extend VMX's #AC handding
Date:   Thu, 30 Jan 2020 07:18:14 -0800
Message-Id: <4A8E14B3-1914-4D0C-A43A-234717179408@amacapital.net>
References: <20200130121939.22383-3-xiaoyao.li@intel.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        x86@kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org
In-Reply-To: <20200130121939.22383-3-xiaoyao.li@intel.com>
To:     Xiaoyao Li <xiaoyao.li@intel.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Jan 30, 2020, at 4:24 AM, Xiaoyao Li <xiaoyao.li@intel.com> wrote:
>=20
> =EF=BB=BFThere are two types of #AC can be generated in Intel CPUs:
> 1. legacy alignment check #AC;
> 2. split lock #AC;
>=20
> Legacy alignment check #AC can be injected to guest if guest has enabled
> alignemnet check.
>=20
> When host enables split lock detection, i.e., split_lock_detect!=3Doff,
> guest will receive an unexpected #AC when there is a split_lock happens in=

> guest since KVM doesn't virtualize this feature to guest.
>=20
> Since the old guests lack split_lock #AC handler and may have split lock
> buges. To make guest survive from split lock, applying the similar policy
> as host's split lock detect configuration:
> - host split lock detect is sld_warn:
>   warning the split lock happened in guest, and disabling split lock
>   detect around VM-enter;
> - host split lock detect is sld_fatal:
>   forwarding #AC to userspace. (Usually userspace dump the #AC
>   exception and kill the guest).

A correct userspace implementation should, with a modern guest kernel, forwa=
rd the exception. Otherwise you=E2=80=99re introducing a DoS into the guest i=
f the guest kernel is fine but guest userspace is buggy.

What=E2=80=99s the intended behavior here?=