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The check was added in some iteration while trying to fix a reported OS X on Core 2 bug, but that bug is elsewhere. The comment is misleading because the guest can call MWAIT with ECX = 0 even if we enforce CPUID5_ECX_INTERRUPT_BREAK; the call would have the exactly the same effect as if the host didn't have the feature. A problem is that a QEMU feature exposes CPUID5_ECX_INTERRUPT_BREAK on CPUs that do not support it. Removing the check changes behavior on last Pentium 4 lines (Presler, Dempsey, and Tulsa, which had VMX and MONITOR while missing INTERRUPT_BREAK) when running a guest OS that uses MWAIT without checking for its presence (QEMU doesn't expose MONITOR). The only known OS that ignores the MONITOR flag is old Mac OS X and we allowed it to bug on Core 2 (MWAIT used to throw #UD and only that OS noticed), so we can save another 20 lines letting it bug on even older CPUs. Alternatively, we can return MWAIT exiting by default and let userspace toggle it. Reviewed-by: Alexander Graf <agraf@suse.de> Acked-by: Borislav Petkov <bp@suse.de> Acked-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Radim Krčmář <rkrcmar@redhat.com> 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.