On Wed, Nov 29, 2017 at 03:24:53PM -0800, H. Peter Anvin wrote: > On 11/29/17 14:31, Borislav Petkov wrote: > > > > A couple of points: > > > > * so this box here has a normal grub installation and apparently grub > > jumps to some other entry point. Ouch. Perhaps you can report this on grub-devel mailing list? And also what version, since I am not sure if this is a distro-specific version? > > > > Yes, Grub as a matter of policy(!) does everything in the most braindead There is a policy on this? Could you point me out to it - it would be enlightening to read it :-) > way possible. You have to use "linux16" or "linuxefi" to make it do > something sane. The Linux bootparams structure is _only_ for Linux. Or are there other OSes that use the same structure to pass information? AFAICT the linuxefi does not exist upstream. > > > * I'm not convinced we need to do everything you typed because this is > > only a temporary issue and once X86_5LEVEL is complete, it should work. > > I mean, it needs to work otherwise forget single-system image and I > > don't think we want to give that up. > > > >> However, if the bootloader jumps straight into the code what do you > >> expect it to do? We have no real concept about what we'd need to do to > >> issue a message as we really don't know what devices are available on > >> the system, etc. If the screen_info field in struct boot_params has > >> been initialized then we actually *do* know how to write to the screen > >> -- if you are okay with including a text font etc. since modern systems > >> boot in graphics mode. > > > > We switch to text mode and dump our message. Can we do that? > > What is text mode? It is hardware that is going away(*), and you don't > even know if you have a display screen on your system at all, or how > you'd have to configure your display hardware even if it is "mostly" VGA. > > > I wouldn't want to do any of this back'n'forth between kernel and boot > > loader because that sounds fragile, at least to me. And again, I'm > > not convinced we should spend too much energy on this as the issue is > > temporary AFAICT. > > Well, it's not just limited to 5-level mode; it's kind a general issue. > We have had this issue for a very, very long time -- all the way back to > i386 PAE at the very least. I'm personally OK with triple-faulting the > CPU in this case. > > -hpa > > > (*) And for good reason -- it is completely memory-latency-bound as you > have an indirect reference for every byte you fetch. In a UMA > system this sucks up an insane amount of system bandwidth, unless > you are willing to burn the area of having a 16K SRAM cache. > > VGA hardware, additionally, has a bunch of insane operations that > have to be memory-mapped. The resulting hardware screws with > pretty much any sane GPU implementation, so I'm fully expecting that > as soon as GPUs no longer come with a CBIOS option ROM VGA hardware > will be dropped more or less immediately. Woot! RIP VGA.. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
