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Re: [RFC Part1 PATCH v3 03/17] x86/mm: Secure Encrypted Virtualization (SEV) support

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  Re: [RFC Part1 PATCH v3 03/17] x86/mm: Secure Encrypted  Virtualization (SEV) support Borislav Petkov <bp@suse.de> - 2017-07-26 06:30 +0200
    Re: [RFC Part1 PATCH v3 03/17] x86/mm: Secure Encrypted  Virtualization (SEV) support Borislav Petkov <bp@suse.de> - 2017-07-27 15:50 +0200

#1696772 — Re: [RFC Part1 PATCH v3 03/17] x86/mm: Secure Encrypted Virtualization (SEV) support

On Mon, Jul 24, 2017 at 02:07:43PM -0500, Brijesh Singh wrote:
> From: Tom Lendacky <thomas.lendacky@amd.com>
> 
> Provide support for Secure Encyrpted Virtualization (SEV). This initial

Your subject misses a verb and patch subjects should have an active verb
denoting what the patch does. The sentence above is a good example.

> support defines a flag that is used by the kernel to determine if it is
> running with SEV active.
> 
> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
> ---
>  arch/x86/include/asm/mem_encrypt.h | 2 ++
>  arch/x86/mm/mem_encrypt.c          | 3 +++
>  include/linux/mem_encrypt.h        | 8 +++++++-
>  3 files changed, 12 insertions(+), 1 deletion(-)

...

> diff --git a/arch/x86/mm/mem_encrypt.c b/arch/x86/mm/mem_encrypt.c
> index 0fbd092..1e4643e 100644
> --- a/arch/x86/mm/mem_encrypt.c
> +++ b/arch/x86/mm/mem_encrypt.c
> @@ -40,6 +40,9 @@ static char sme_cmdline_off[] __initdata = "off";
>  unsigned long sme_me_mask __section(.data) = 0;
>  EXPORT_SYMBOL_GPL(sme_me_mask);
>  
> +unsigned int sev_enabled __section(.data) = 0;
> +EXPORT_SYMBOL_GPL(sev_enabled);

So sev_enabled is a pure bool used only in bool context, not like
sme_me_mask whose value is read too. Which means, you can make the
former static and query it only through accessor functions.

>  /* Buffer used for early in-place encryption by BSP, no locking needed */
>  static char sme_early_buffer[PAGE_SIZE] __aligned(PAGE_SIZE);
>  
> diff --git a/include/linux/mem_encrypt.h b/include/linux/mem_encrypt.h
> index 1255f09..ea0831a 100644
> --- a/include/linux/mem_encrypt.h
> +++ b/include/linux/mem_encrypt.h
> @@ -22,12 +22,18 @@
>  #else	/* !CONFIG_ARCH_HAS_MEM_ENCRYPT */
>  
>  #define sme_me_mask	0UL
> +#define sev_enabled	0
>  
>  #endif	/* CONFIG_ARCH_HAS_MEM_ENCRYPT */
>  
>  static inline bool sme_active(void)
>  {
> -	return !!sme_me_mask;
> +	return (sme_me_mask && !sev_enabled);

You don't need the brackets. Below too.

> +}
> +
> +static inline bool sev_active(void)
> +{
> +	return (sme_me_mask && sev_enabled);
>  }

So this is confusing, TBH. SME and SEV are not mutually exclusive and
yet the logic here says so. Why?

I mean, in the hypervisor context, sme_active() is still true.

/me is confused.

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
-- 

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#1697995

On Wed, Jul 26, 2017 at 11:47:32AM -0500, Tom Lendacky wrote:
> If it's made static then the sme_active()/sev_active() inline functions
> would need to be turned into functions within the mem_encrypt.c file. So
> there's a trade-off to do that, which is the better one?

Simple: why do we have functions if the variables are exported?

The reasoning for sme_me_mask is more or less obvious but for sev_enabled...

IOW, either make the bool static and unlinine the function - this way
you're free to change how you determine whether SEV is enabled later as
callers will be using the function.

Or, if it doesn't really matter because you can always change callers
later, simply drop sev_active() the function and use a bool sev_active
everywhere.

> The kernel needs to distinguish between running under SME and running
> under SEV. SME and SEV are similar but not the same. The trampoline code
> is a good example.  Before paging is activated, SME will access all
> memory as decrypted, but SEV will access all memory as encrypted.  So
> when APs are being brought up under SME the trampoline area cannot be
> encrypted, whereas under SEV the trampoline area must be encrypted.

I guess you're sensing by now that we need this clarification in a
comment above it...

:-)

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
-- 

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