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x86/boot/compressed/64: Explain paging_prepare()'s return value 

paging_prepare() returns a two-quadword structure which lands
into RDX:RAX:

  - Address of the trampoline is returned in RAX.
  - Non zero RDX means trampoline needs to enable 5-level paging.

Document that explicitly.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: dave.hansen@linux.intel.com
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Kyle D Pelton <kyle.d.pelton@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Wei Huang <wei@redhat.com>
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/20190206154756.matwldebbxkmlnae@black.fi.intel.com
This commit is contained in:
Kirill A. Shutemov 2019-02-06 18:29:08 +03:00 committed by Borislav Petkov
parent 8ad382dd11
commit 82434d23f3
1 changed files with 6 additions and 3 deletions
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9
arch/x86/boot/compressed/head_64.S
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@ -358,8 +358,11 @@ ENTRY(startup_64)
* paging_prepare() sets up the trampoline and checks if we need to
* enable 5-level paging.
*
* Address of the trampoline is returned in RAX.
* Non zero RDX on return means we need to enable 5-level paging.
* paging_prepare() returns a two-quadword structure which lands
* into RDX:RAX:
* - Address of the trampoline is returned in RAX.
* - Non zero RDX means trampoline needs to enable 5-level
* paging.
*
* RSI holds real mode data and needs to be preserved across
* this function call.
@ -565,7 +568,7 @@ adjust_got:
*
* RDI contains the return address (might be above 4G).
* ECX contains the base address of the trampoline memory.
* Non zero RDX on return means we need to enable 5-level paging.
* Non zero RDX means trampoline needs to enable 5-level paging.
*/
ENTRY(trampoline_32bit_src)
/* Set up data and stack segments */