linux-kernel - Re: [PATCH v3 1/5] x86/boot/compressed/64: Introduce sev

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Message-ID: <20201026182706.GG22116@zn.tnic>
Date:   Mon, 26 Oct 2020 19:27:06 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Joerg Roedel <joro@...tes.org>
Cc:     x86@...nel.org, Joerg Roedel <jroedel@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Martin Radev <martin.b.radev@...il.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/5] x86/boot/compressed/64: Introduce sev_status

On Wed, Oct 21, 2020 at 02:39:34PM +0200, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@...e.de>
> 
> Introduce sev_status and initialize it together with sme_me_mask to have
> an indicator which SEV features are enabled.
> 
> Signed-off-by: Joerg Roedel <jroedel@...e.de>
> ---
>  arch/x86/boot/compressed/mem_encrypt.S | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/boot/compressed/mem_encrypt.S b/arch/x86/boot/compressed/mem_encrypt.S
> index dd07e7b41b11..2192b3bd78d8 100644
> --- a/arch/x86/boot/compressed/mem_encrypt.S
> +++ b/arch/x86/boot/compressed/mem_encrypt.S
> @@ -81,6 +81,13 @@ SYM_FUNC_START(set_sev_encryption_mask)
>  
>  	bts	%rax, sme_me_mask(%rip)	/* Create the encryption mask */
>  
> +	/* Read sev_status */
> +	movl	$MSR_AMD64_SEV, %ecx
> +	rdmsr
> +	shlq	$32, %rdx
> +	orq	%rdx, %rax
> +	movq	%rax, sev_status(%rip)

A couple of lines above you call get_sev_encryption_bit() which already
reads MSR_AMD64_SEV. Why not set sev_status there too instead of reading
that MSR again here?

It can read that MSR once and use sev_status(%rip) from then on to avoid
reading that MSR multiple times...

-- 
Regards/Gruss,
    Boris.

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