Re: [PATCH] x86/boot/compressed/64: Do not corrupt EDX on EFER.LME=1 setting

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

On Wed, Feb 06, 2019 at 03:21:41PM +0000, Borislav Petkov wrote:
> On Wed, Feb 06, 2019 at 02:52:53PM +0300, Kirill A. Shutemov wrote:
> > RDMSR in the trampoline code overrides EDX, but we use the register to
> > indicate if 5-level paging has to enabled. It leads to failure to boot
> > on a 5-level paging machine.
> > 
> > Preserve EDX on the stack while we are dealing with EFER.
> 
> Comment says:
> 
>  * Non zero RDX on return means we need to enable 5-level paging.
> 
> Is that per-chance refering to struct paging_config which
> paging_prepare() returns and on that return rdx contains
> paging_config.l5_required which is 1 when 5 level is to be enabled?

Yes.

> If so, is that written somewhere explicitly? Because it is not
> immediately clear at least to me why that RDX is live...

What about this:

From d43b9d157c574baf782f6d9982fe6f2c1f918c0e Mon Sep 17 00:00:00 2001
From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Date: Wed, 6 Feb 2019 18:29:08 +0300
Subject: [PATCH] x86/boot/compressed/64: Fix confusing comment for
 paging_config()

paging_prepare() returns 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.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 arch/x86/boot/compressed/head_64.S | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index f62e347862cc..87509a3f00f4 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -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 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 */
-- 
 Kirill A. Shutemov