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

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Date:   Wed, 21 Jun 2017 09:16:05 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Tom Lendacky <thomas.lendacky@....com>
cc:     linux-arch@...r.kernel.org, linux-efi@...r.kernel.org,
        kvm@...r.kernel.org, linux-doc@...r.kernel.org, x86@...nel.org,
        kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kasan-dev@...glegroups.com, xen-devel@...ts.xen.org,
        linux-mm@...ck.org, iommu@...ts.linux-foundation.org,
        Brijesh Singh <brijesh.singh@....com>,
        Toshimitsu Kani <toshi.kani@....com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Alexander Potapenko <glider@...gle.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Larry Woodman <lwoodman@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Joerg Roedel <joro@...tes.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Dave Young <dyoung@...hat.com>, Rik van Riel <riel@...hat.com>,
        Arnd Bergmann <arnd@...db.de>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Juergen Gross <jgross@...e.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v7 08/36] x86/mm: Add support to enable SME in early boot
 processing

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