On Fri, Jun 14, 2024 at 09:06:30AM -0500, Tom Lendacky wrote: > On 6/13/24 09:56, Borislav Petkov wrote: > > On Thu, Jun 13, 2024 at 04:41:00PM +0300, Kirill A. Shutemov wrote: > > > It is easy enough to do. See the patch below. > > > > Thanks, will have a look. > > > > > But I am not sure if I can justify it properly. If someone doesn't really > > > need 5-level paging, disabling it at compile-time would save ~34K of > > > kernel code with the configuration. > > > > > > Is it worth saving ~100 lines of code? > > > > Well, it goes both ways: is it worth saving ~34K kernel text and for that make > > the code a lot less conditional, more readable, contain less ugly ifdeffery, > > Won't getting rid of the config option cause 5-level to be used by default > on all platforms that support it? The no5lvl command line option would have > to be used to get 4-level paging at that point. Yes, there won't be compile-time option to disable 5-level paging. Is it a problem? We benchmarked it back when 5-level paging got introduced and were not able to see a measurable difference between 4- and 5-level paging on the same machine. There's some memory overhead on more page table, but it shouldn't be a show stopper. I would prefer to get 5-level paging enabled if the machine supports it. "no5lvl" cmdline option can be useful for debug or if your workload is somehow special. -- Kiryl Shutsemau / Kirill A. Shutemov
