Messages in this thread | | | Subject | Re: [PATCH v2] KVM: VMX: Enable Notify VM exit | From | Xiaoyao Li <> | Date | Mon, 7 Jun 2021 17:24:30 +0800 |
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On 6/3/2021 9:35 PM, Jim Mattson wrote: > On Wed, Jun 2, 2021 at 6:25 PM Xiaoyao Li <xiaoyao.li@intel.com> wrote: >> >> On 6/2/2021 6:31 PM, Vitaly Kuznetsov wrote: >>> Tao Xu <tao3.xu@intel.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@intel.com> >>>> Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com> >>>> Signed-off-by: Tao Xu <tao3.xu@intel.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? >>> >> >> yes. If we expose it to nested, it should reflect the Notify VM exit to >> L1 when L1 enables it. >> >> But regarding nested, there are more things need to be discussed. e.g., >> 1) It has dependence between L0 and L1, for security consideration. When >> L0 enables it, it shouldn't be turned off during L2 VM is running. >> a. Don't expose to L1 but enable for L1 when L2 VM is running. >> b. expose it to L1 and force it enabled. >> >> 2) When expose it to L1, vmcs02.notify_window needs to be >> min(L0.notify_window, L1.nofity_window) > > I don't think this can be a simple 'min', since L1's clock may run at > a different frequency from L0's clock. >
Good catch. We will take it into account.
thanks!
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