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From:   Andy Lutomirski <luto@amacapital.net>
Mime-Version: 1.0 (1.0)
Subject: Re: [PATCH 2/2] KVM: VMX: Extend VMX's #AC handding
Date:   Fri, 31 Jan 2020 13:33:17 -0800
Message-Id: <E1F9CE39-7D61-43E1-B871-6D4BFA4B6D66@amacapital.net>
References: <20200131210424.GG18946@linux.intel.com>
Cc:     Xiaoyao Li <xiaoyao.li@intel.com>,
        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>, x86@kernel.org,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org
In-Reply-To: <20200131210424.GG18946@linux.intel.com>
To:     Sean Christopherson <sean.j.christopherson@intel.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Jan 31, 2020, at 1:04 PM, Sean Christopherson <sean.j.christopherson@in=
tel.com> wrote:
>=20
> =EF=BB=BFOn Fri, Jan 31, 2020 at 12:57:51PM -0800, Andy Lutomirski wrote:
>>=20
>>>> On Jan 31, 2020, at 12:18 PM, Sean Christopherson <sean.j.christopherso=
n@intel.com> wrote:
>>>=20
>>> This is essentially what I proposed a while back.  KVM would allow enabl=
ing
>>> split-lock #AC in the guest if and only if SMT is disabled or the enable=
 bit
>>> is per-thread, *or* the host is in "warn" mode (can live with split-lock=
 #AC
>>> being randomly disabled/enabled) and userspace has communicated to KVM t=
hat
>>> it is pinning vCPUs.
>>=20
>> How about covering the actual sensible case: host is set to fatal?  In th=
is
>> mode, the guest gets split lock detection whether it wants it or not. How=
 do
>> we communicate this to the guest?
>=20
> KVM doesn't advertise split-lock #AC to the guest and returns -EFAULT to t=
he
> userspace VMM if the guest triggers a split-lock #AC.
>=20
> Effectively the same behavior as any other userspace process, just that KV=
M
> explicitly returns -EFAULT instead of the process getting a SIGBUS.


Which helps how if the guest is actually SLD-aware?

I suppose we could make the argument that, if an SLD-aware guest gets #AC at=
 CPL0, it=E2=80=99s a bug, but it still seems rather nicer to forward the #A=
C to the guest instead of summarily killing it.

ISTM, on an SLD-fatal host with an SLD-aware guest, the host should tell the=
 guest =E2=80=9Chey, you may not do split locks =E2=80=94 SLD is forced on=E2=
=80=9D and the guest should somehow acknowledge it so that it sees the archi=
tectural behavior instead of something we made up.  Hence my suggestion.=