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 17:38:04 +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,
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        Brijesh Singh <brijesh.singh@....com>,
        Toshimitsu Kani <toshi.kani@....com>,
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        Matt Fleming <matt@...eblueprint.co.uk>,
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        "H. Peter Anvin" <hpa@...or.com>,
        Larry Woodman <lwoodman@...hat.com>,
        Jonathan Corbet <corbet@....net>,
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        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 Wed, 21 Jun 2017, Tom Lendacky wrote:
> On 6/21/2017 2:16 AM, Thomas Gleixner wrote:
> > 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?

See below.

> > 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().

That's much preferred. And the return value wants to be documented in both
C and ASM code.

> 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).

        call verify_cpu

        movq    $(init_top_pgt - __START_KERNEL_map), %rax

So if you make that:

	/*
	 * Sanitize CPU configuration and retrieve the modifier
	 * for the initial pgdir entry which will be programmed
	 * into CR3. Depends on enabled SME encryption, normally 0.
	 */
	call __startup_secondary_64

        addq    $(init_top_pgt - __START_KERNEL_map), %rax

You can hide that stuff in C-code nicely without adding any cruft to the
ASM code.

Thanks,

	tglx