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 10:14:35 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Thomas Gleixner <tglx@...utronix.de>
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 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
>