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

[Wei Huang]

On 2/6/19 5:52 AM, 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.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Fixes: b677dfae5aa1 ("x86/boot/compressed/64: Set EFER.LME=1 in 32-bit trampoline before returning to long mode")
> Reported-by: Kyle D Pelton <kyle.d.pelton@xxxxxxxxx>
> Cc: Wei Huang <wei@xxxxxxxxxx>
> ---
>  arch/x86/boot/compressed/head_64.S | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> index f105ae8651c9..f62e347862cc 100644
> --- a/arch/x86/boot/compressed/head_64.S
> +++ b/arch/x86/boot/compressed/head_64.S
> @@ -602,10 +602,12 @@ ENTRY(trampoline_32bit_src)
>  3:
>  	/* Set EFER.LME=1 as a precaution in case hypervsior pulls the rug */
>  	pushl	%ecx
> +	pushl	%edx
>  	movl	$MSR_EFER, %ecx
>  	rdmsr
>  	btsl	$_EFER_LME, %eax
>  	wrmsr
> +	popl	%edx
>  	popl	%ecx
>  
>  	/* Enable PAE and LA57 (if required) paging modes */
> 

Oops, rdmsr indeed corrupts EDX.

Acked-by: Wei Huang <wei@xxxxxxxxxx>