Tao Xu <tao3.xu@xxxxxxxxx> writes: > On 6/2/21 6:31 PM, Vitaly Kuznetsov wrote: >> Tao Xu <tao3.xu@xxxxxxxxx> 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@xxxxxxxxx> >>> Signed-off-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx> >>> Signed-off-by: Tao Xu <tao3.xu@xxxxxxxxx> >>> --- >>> >>> 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
