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

[Tom Lendacky <thomas.lendacky@xxxxxxx>]

On 6/21/2017 2:16 AM, Thomas Gleixner wrote:
> 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?
I made it unconditional because of the call from head_64.S. I can't make
use of the C level static inline function and since the mask is not a
variable if CONFIG_AMD_MEM_ENCRYPT is not configured (#defined to 0) I
can't reference the variable directly.

I could create a #define in head_64.S that changes this to load rax with
the variable if CONFIG_AMD_MEM_ENCRYPT is configured or a zero if it's
not or add a #ifdef at that point in the code directly. Thoughts on
that?

> > 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?
I was trying to keep it explicit as to what was happening, but I can
move those calls into __startup_64(). I'll still need the call to
sme_get_me_mask() in the secondary_startup_64 path, though (depending on
your thoughts to the above response).

> > @@ -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.
Same as above, this can be updated based on the decided approach.

Thanks,
Tom

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