Re: [Xen-devel] [PATCH v7 08/36] x86/mm: Add support to enable SME in early boot processing

[Thomas Gleixner <tglx@xxxxxxxxxxxxx>]

On Fri, 16 Jun 2017, Tom Lendacky wrote:
> diff --git a/arch/x86/include/asm/mem_encrypt.h 
> b/arch/x86/include/asm/mem_encrypt.h
> index a105796..988b336 100644
> --- a/arch/x86/include/asm/mem_encrypt.h
> +++ b/arch/x86/include/asm/mem_encrypt.h
> @@ -15,16 +15,24 @@
>  
>  #ifndef __ASSEMBLY__
>  
> +#include <linux/init.h>
> +
>  #ifdef CONFIG_AMD_MEM_ENCRYPT
>  
>  extern unsigned long sme_me_mask;
>  
> +void __init sme_enable(void);
> +
>  #else        /* !CONFIG_AMD_MEM_ENCRYPT */
>  
>  #define sme_me_mask  0UL
>  
> +static inline void __init sme_enable(void) { }
> +
>  #endif       /* CONFIG_AMD_MEM_ENCRYPT */
>  
> +unsigned long sme_get_me_mask(void);

Why is this an unconditional function? Isn't the mask simply 0 when the MEM
ENCRYPT support is disabled?

> diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
> index 6225550..ef12729 100644
> --- a/arch/x86/kernel/head_64.S
> +++ b/arch/x86/kernel/head_64.S
> @@ -78,7 +78,29 @@ startup_64:
>       call    __startup_64
>       popq    %rsi
>  
> -     movq    $(early_top_pgt - __START_KERNEL_map), %rax
> +     /*
> +      * Encrypt the kernel if SME is active.
> +      * The real_mode_data address is in %rsi and that register can be
> +      * clobbered by the called function so be sure to save it.
> +      */
> +     push    %rsi
> +     call    sme_encrypt_kernel
> +     pop     %rsi

That does not make any sense. Neither the call to sme_encrypt_kernel() nor
the following call to sme_get_me_mask().

__startup_64() is already C code, so why can't you simply call that from
__startup_64() in C and return the mask from there?

> @@ -98,7 +120,20 @@ ENTRY(secondary_startup_64)
>       /* Sanitize CPU configuration */
>       call verify_cpu
>  
> -     movq    $(init_top_pgt - __START_KERNEL_map), %rax
> +     /*
> +      * Get the SME encryption mask.
> +      *  The encryption mask will be returned in %rax so we do an ADD
> +      *  below to be sure that the encryption mask is part of the
> +      *  value that will stored in %cr3.
> +      *
> +      * The real_mode_data address is in %rsi and that register can be
> +      * clobbered by the called function so be sure to save it.
> +      */
> +     push    %rsi
> +     call    sme_get_me_mask
> +     pop     %rsi

Do we really need a call here? The mask is established at this point, so
it's either 0 when the encryption stuff is not compiled in or it can be
retrieved from a variable which is accessible at this point.

> +
> +     addq    $(init_top_pgt - __START_KERNEL_map), %rax
>  1:
>  
>       /* Enable PAE mode, PGE and LA57 */

Thanks,

        tglx

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