linux-kernel - Re: [PATCH] x86/boot/compressed/64: Do not corrupt EDX on EFER.LME=1 setting

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Message-ID: <20190206152141.GF7314@zn.tnic>
Date:   Wed, 6 Feb 2019 16:21:41 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
        dave.hansen@...ux.intel.com, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Kyle D Pelton <kyle.d.pelton@...el.com>,
        Wei Huang <wei@...hat.com>
Subject: Re: [PATCH] x86/boot/compressed/64: Do not corrupt EDX on EFER.LME=1
 setting

On Wed, Feb 06, 2019 at 02:52:53PM +0300, Kirill A. Shutemov wrote:
> RDMSR in the trampoline code overrides EDX, but we use the register to
> indicate if 5-level paging has to enabled. It leads to failure to boot
> on a 5-level paging machine.
> 
> Preserve EDX on the stack while we are dealing with EFER.

Comment says:

 * Non zero RDX on return means we need to enable 5-level paging.

Is that per-chance refering to struct paging_config which
paging_prepare() returns and on that return rdx contains
paging_config.l5_required which is 1 when 5 level is to be enabled?

If so, is that written somewhere explicitly? Because it is not
immediately clear at least to me why that RDX is live...

Thx.

-- 
Regards/Gruss,
    Boris.

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