Re: [PATCH 4.4 00/37] 4.4.110-stable review

[Thomas Gleixner]

On Thu, 11 Jan 2018, Thomas Gleixner wrote:
> On Thu, 11 Jan 2018, Linus Torvalds wrote:
> 
> > On Thu, Jan 11, 2018 at 12:37 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> > >
> > > 67a9108ed431 ("x86/efi: Build our own page table structures")
> > >
> > > got rid of EFI depending on real_mode_header->trampoline_pgd
> > 
> > So I think it only got rid of by default - the codepath is still
> > there, the allocation is still there, it's just that it's not actually
> > used unless somebody does that "efi=old_mmap" thing.
> 
> Yes, the trampoline_pgd is still around, but I can't figure out how it
> would be used after boot. Confused, digging more.

So coming back to the same commit. From the changelog:

    This is caused by mapping EFI regions with RWX permissions.
    There isn't much we can do to restrict the permissions for these
    regions due to the way the firmware toolchains mix code and
    data, but we can at least isolate these mappings so that they do
    not appear in the regular kernel page tables.
    
    In commit d2f7cbe7b26a ("x86/efi: Runtime services virtual
    mapping") we started using 'trampoline_pgd' to map the EFI
    regions because there was an existing identity mapping there
    which we use during the SetVirtualAddressMap() call and for
    broken firmware that accesses those addresses.

So this very commit gets rid of the (ab)use of trampoline_pgd and allocates
efi_pgd, which we made use the proper size.

trampoline_pgd is since then only used to get into long mode in
realmode/rm/trampoline_64.S and for reboot in machine_real_restart().

The runtime services stuff does not use it in kernel versions >= 4.6

Thanks,

	tglx