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The VMX-preemption timer is used by KVM as a way to set deadlines for the guest (i.e. timer emulation). That was safe till very recently when capability KVM_X86_DISABLE_EXITS_MWAIT to disable intercepting MWAIT was introduced. According to Intel SDM 25.5.1: """ The VMX-preemption timer operates in the C-states C0, C1, and C2; it also operates in the shutdown and wait-for-SIPI states. If the timer counts down to zero in any state other than the wait-for SIPI state, the logical processor transitions to the C0 C-state and causes a VM exit; the timer does not cause a VM exit if it counts down to zero in the wait-for-SIPI state. The timer is not decremented in C-states deeper than C2. """ Now once the guest issues the MWAIT with a c-state deeper than C2 the preemption timer will never wake it up again since it stopped ticking! Usually this is compensated by other activities in the system that would wake the core from the deep C-state (and cause a VMExit). For example, if the host itself is ticking or it received interrupts, etc! So disable the VMX-preemption timer if MWAIT is exposed to the guest! Cc: Paolo Bonzini <pbonzini@redhat.com> Cc: Radim Krčmář <rkrcmar@redhat.com> Cc: kvm@vger.kernel.org Signed-off-by: KarimAllah Ahmed <karahmed@amazon.de> Fixes: 4d5422cea3b61f158d58924cbb43feada456ba5c Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.