When Linux runs as a guest OS, CPUID is privileged instruction, sync_core() will be very slow. If apply this patch, 200ms will be saved for kernel initial, when Linux runs as a guest OS. BR. Ning. -----Original Message----- From: gregkh@xxxxxxxxxxxxxxxxxxx [mailto:gregkh@xxxxxxxxxxxxxxxxxxx] Sent: Wednesday, January 24, 2018 5:01 PM To: Zhang, Ning A <ning.a.zhang@xxxxxxxxx> Cc: Andy Lutomirski <luto@xxxxxxxxxx>; tglx@xxxxxxxxxxxxx; stable@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx Subject: Re: backport Rewrite sync_core() to use IRET-to-self to stable kernels? On Wed, Jan 24, 2018 at 08:19:44AM +0000, Zhang, Ning A wrote: > Thanks Greg, Andy > > How about c198b121b1a1d7a7171770c634cd49191bac4477? Why do you think that is needed in the older stable kernels? What problem does it solve? Do any distros have this patch in their kernels? thanks, greg k-h