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From: Xiaoyao Li <xiaoyao.li@intel.com>
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
Subject: Re: [PATCH 2/2] KVM: VMX: Extend VMX's #AC handding
Date: Sun, 2 Feb 2020 00:58:15 +0800	[thread overview]
Message-ID: <b2e2310d-2228-45c2-8174-048e18a46bb6@intel.com> (raw)
In-Reply-To: <E1F9CE39-7D61-43E1-B871-6D4BFA4B6D66@amacapital.net>

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.
> 


  reply	other threads:[~2020-02-01 16:59 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-30 12:19 [PATCH 0/2] kvm: split_lock: Fix emulator and extend #AC handler Xiaoyao Li
2020-01-30 12:19 ` [PATCH 1/2] KVM: x86: Emulate split-lock access as a write Xiaoyao Li
2020-01-30 12:31   ` David Laight
2020-01-30 15:16     ` Andy Lutomirski
2020-01-31 20:01       ` Sean Christopherson
2020-02-04 14:47         ` Vitaly Kuznetsov
2020-02-10 21:59           ` Sean Christopherson
2020-01-30 12:19 ` [PATCH 2/2] KVM: VMX: Extend VMX's #AC handding Xiaoyao Li
2020-01-30 15:18   ` Andy Lutomirski
2020-01-30 16:29     ` Xiaoyao Li
2020-01-30 17:16       ` Andy Lutomirski
2020-01-31  7:22         ` Xiaoyao Li
2020-01-31 15:37           ` Andy Lutomirski
2020-01-31 17:47             ` Xiaoyao Li
2020-01-31 20:17               ` Sean Christopherson
2020-01-31 20:57                 ` Andy Lutomirski
2020-01-31 21:04                   ` Sean Christopherson
2020-01-31 21:33                     ` Andy Lutomirski
2020-02-01 16:58                       ` Xiaoyao Li [this message]
2020-02-01 17:56                         ` Andy Lutomirski
2020-02-02  4:33                           ` Xiaoyao Li
2020-02-03 18:49                             ` Andy Lutomirski
2020-02-04  6:03                               ` Sean Christopherson

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