On Sun, 27 Sep, at 12:40:14PM, Borislav Petkov wrote: > On Sun, Sep 27, 2015 at 09:06:44AM +0200, Ingo Molnar wrote: > > Could we please re-list all the arguments pro and contra of 1:1 physical mappings, > > in a post that also explains the background so that more people can chime in, not > > just people versed in EFI internals? It's very much possible that a bad decision > > was made. > > The main reason why we did the additional, top-down mapping was kexec > kernel wanting to use UEFI runtime facilities too and the braindead > design of SetVirtualAddressMap() being callable only once per system > boot. So we had to have stable mappings which are valid in the kexec-ed > kernel too. > > But this was long time ago and I most certainly have forgotten all the > details. That's a pretty good summary for x86. I think specifically the reason we map the EFI memmap entries "backwards" (entry N has higher VA than entry N+1) is because the code was easier to write that way, but you'll know better than me ;-) > And now I'm wondering why didn't we do the 1:1 thing and rebuild the > exact same EFI pagetable in the kexec-ed kernel? Because when we do > an EFI call, we switch to the special pagetable so why didn't we make > the kexec-ed kernel rebuild the 1:1 pagetable which it can use for EFI > calls... > > Hmm, again, I've forgotten a lot of details so I'm sure Matt will come > in and say "No, you can't do that because..." I *think* the only reason was the Apple firmware problem where it explodes if you pass the 1:1 mappings to SetVirtualAddressMap(). And obviously people do want to use kexec with Apple machines. It's probably worth revisiting this whole thing from the x86 side. -- Matt Fleming, Intel Open Source Technology Center -- To unsubscribe from this list: send the line "unsubscribe linux-efi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
