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Subject: Re: [PATCH 2/2] KVM: VMX: Extend VMX's #AC handding
To:     Andy Lutomirski <luto@amacapital.net>,
        Sean Christopherson <sean.j.christopherson@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>, x86@kernel.org,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org
References: <20200131210424.GG18946@linux.intel.com>
 <E1F9CE39-7D61-43E1-B871-6D4BFA4B6D66@amacapital.net>
From:   Xiaoyao Li <xiaoyao.li@intel.com>
Message-ID: <b2e2310d-2228-45c2-8174-048e18a46bb6@intel.com>
Date:   Sun, 2 Feb 2020 00:58:15 +0800
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On 2/1/2020 5:33 AM, Andy Lutomirski wrote:
> 
> 
>> On Jan 31, 2020, at 1:04 PM, Sean Christopherson <sean.j.christopherson@intel.com> wrote:
>>
>> ﻿On Fri, Jan 31, 2020 at 12:57:51PM -0800, Andy Lutomirski wrote:
>>>
>>>>> On Jan 31, 2020, at 12:18 PM, Sean Christopherson <sean.j.christopherson@intel.com> wrote:
>>>>
>>>> This is essentially what I proposed a while back.  KVM would allow enabling
>>>> 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 that
>>>> it is pinning vCPUs.
>>>
>>> How about covering the actual sensible case: host is set to fatal?  In this
>>> mode, the guest gets split lock detection whether it wants it or not. How do
>>> we communicate this to the guest?
>>
>> KVM doesn't advertise split-lock #AC to the guest and returns -EFAULT to the
>> userspace VMM if the guest triggers a split-lock #AC.
>>
>> Effectively the same behavior as any other userspace process, just that KVM
>> 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’s a bug, but it still seems rather nicer to forward the #AC to the guest instead of summarily killing it.

If KVM does advertise split-lock detection to the guest, then kvm/host 
can know whether a guest is SLD-aware by checking guest's 
MSR_TEST_CTRL.SPLIT_LOCK_DETECT bit.

  - If guest's MSR_TEST_CTRL.SPLIT_LOCK_DETECT is set, it indicates 
guest is SLD-aware so KVM forwards #AC to guest.

  - If not set. It may be a old guest or a malicious guest or a guest 
without SLD support, and we cannot figure it out. So we have to kill the 
guest when host is SLD-fatal, and let guest survive when SLD-WARN for 
old sane buggy guest.

In a word, all the above is on the condition that KVM advertise 
split-lock detection to guest. But this patch doesn't do this. Maybe I 
should add that part in v2.

> ISTM, on an SLD-fatal host with an SLD-aware guest, the host should tell the guest “hey, you may not do split locks — SLD is forced on” and the guest should somehow acknowledge it so that it sees the architectural behavior instead of something we made up.  Hence my suggestion.
>