* Kirill A. Shutemov <kirill.shute...@linux.intel.com> wrote:
> [
> The patch is based on my boot-time switching patchset and would not apply
> directly to current upstream, but I would appreciate early feedback.
> ]
>
> This patch addresses shortcoming in current boot process on machines
> that supports 5-level paging.
>
> If bootloader enables 64-bit mode with 4-level paging, we need to
> switch over to 5-level paging. The switching requires disabling paging.
> It works fine if kernel itself is loaded below 4G.
>
> If bootloader put the kernel above 4G (not sure if anybody does this),
> we would loose control as soon as paging is disabled as code becomes
> unreachable.
>
> This patch implements trampoline in lower memory to handle this
> situation.
>
> Apart from trampoline itself we also need place to store top level page
> table in lower memory as we don't have a way to load 64-bit value into
> CR3 from 32-bit mode. We only really need 8-bytes there as we only use
> the very first entry of the page table. but we allocate whole page
> anyway. We cannot have the code in the same because, there's hazard that
> a CPU would read page table speculatively and get confused seeing
> garbage.
>
> We only need the memory for very short time, until main kernel image
> setup its own page tables.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shute...@linux.intel.com>
> ---
> arch/x86/boot/compressed/head_64.S | 83
> ++++++++++++++++++++++--------------
> arch/x86/boot/compressed/pagetable.c | 45 +++++++++++++++++++
> arch/x86/boot/compressed/pagetable.h | 16 +++++++
> 3 files changed, 111 insertions(+), 33 deletions(-)
> create mode 100644 arch/x86/boot/compressed/pagetable.h
Yeah, things like this is what I'd like to see, but could we please structure
it a
bit differently. Splitting it up more is very important, as 100+ lines
difficult
commits are difficult to debug after the fact. I'd suggest the following
split-up:
patch 1: introduce place_trampoline(), call it from the assembly - but don't
do anything
patch 2: move as much existing assembly code to C code in place_trampoline()
as possible
patch 3: modify remaining assembly code for dynamic 5-level pagetable support
patch 4: modify place_trampoline() for dynamic 5-level pagetable support
Also, if you can think of more boot code in that file to move to C reasonably,
please do it, even if it's unrelated to 5-level paging at the moment. The more
boot assembly code you manage to move to C, the better x86 maintainers will
like
your patch-set. We are easy to corrupt with such patches! :-)
Thanks,
Ingo
