linux-kernel - Re: [PATCH v2] KVM: VMX: Enable Notify VM exit

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Date:   Thu, 03 Jun 2021 15:43:17 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Tao Xu <tao3.xu@...el.com>
Cc:     x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Xiaoyao Li <xiaoyao.li@...el.com>, pbonzini@...hat.com,
        seanjc@...gle.com, wanpengli@...cent.com, jmattson@...gle.com,
        joro@...tes.org, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, hpa@...or.com
Subject: Re: [PATCH v2] KVM: VMX: Enable Notify VM exit

Tao Xu <tao3.xu@...el.com> writes:

> On 6/2/21 6:31 PM, Vitaly Kuznetsov wrote:
>> Tao Xu <tao3.xu@...el.com> writes:
>> 
>>> There are some cases that malicious virtual machines can cause CPU stuck
>>> (event windows don't open up), e.g., infinite loop in microcode when
>>> nested #AC (CVE-2015-5307). No event window obviously means no events,
>>> e.g. NMIs, SMIs, and IRQs will all be blocked, may cause the related
>>> hardware CPU can't be used by host or other VM.
>>>
>>> To resolve those cases, it can enable a notify VM exit if no event
>>> window occur in VMX non-root mode for a specified amount of time
>>> (notify window). Since CPU is first observed the risk of not causing
>>> forward progress, after notify window time in a units of crystal clock,
>>> Notify VM exit will happen. Notify VM exit can happen incident to delivery
>>> of a vectored event.
>>>
>>> Expose a module param for configuring notify window, which is in unit of
>>> crystal clock cycle.
>>> - A negative value (e.g. -1) is to disable this feature.
>>> - Make the default as 0. It is safe because an internal threshold is added
>>> to notify window to ensure all the normal instructions being coverd.
>>> - User can set it to a large value when they want to give more cycles to
>>> wait for some reasons, e.g., silicon wrongly kill some normal instruction
>>> due to internal threshold is too small.
>>>
>>> Notify VM exit is defined in latest Intel Architecture Instruction Set
>>> Extensions Programming Reference, chapter 9.2.
>>>
>>> Co-developed-by: Xiaoyao Li <xiaoyao.li@...el.com>
>>> Signed-off-by: Xiaoyao Li <xiaoyao.li@...el.com>
>>> Signed-off-by: Tao Xu <tao3.xu@...el.com>
>>> ---
>>>
>>> Changelog:
>>> v2:
>>>       Default set notify window to 0, less than 0 to disable.
>>>       Add more description in commit message.
>> 
>> Sorry if this was already discussed, but in case of nested
>> virtualization and when L1 also enables
>> SECONDARY_EXEC_NOTIFY_VM_EXITING, shouldn't we just reflect NOTIFY exits
>> during L2 execution to L1 instead of crashing the whole L1?
>> 
> Notify VM Exit will not crash L1 guest if VM context valid in exit 
> qualification. After VM exit, VMM can resume the guest normally.

Wrong choice of words, sorry. Indeed, VMM is free to decide what to do
upon such vmexit.

-- 
Vitaly