linux-kernel - Re: [PATCH] x86/mm: Create an SME workarea in the kernel for early encryption

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Date:   Wed, 12 Jun 2019 17:46:07 +0000
From:   "Lendacky, Thomas" <Thomas.Lendacky@....com>
To:     Dave Hansen <dave.hansen@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Baoquan He <bhe@...hat.com>, Lianbo Jiang <lijiang@...hat.com>
Subject: Re: [PATCH] x86/mm: Create an SME workarea in the kernel for early
 encryption

On 6/12/19 10:00 AM, Dave Hansen wrote:
> On 6/12/19 6:32 AM, Lendacky, Thomas wrote:
>> Create a section for SME in the vmlinux.lds.S.  Position it after "_end"
>> so that the memory will be reclaimed during boot and, since it is all
>> zeroes, it compresses well. 
> 
> I don't think I realized that things after _end get reclaimed.  Do we do
> that at the same spot that we do init data or somewhere else?

I was looking at the start of setup_arch() in arch/x86/kernel/setup.c,
where there's a memblock_reserve() done for the kernel (it reserves from
_text to __bss_stop, not all the way to _end, and later the brk area
is reserved). At that point, my take was that the memory outside the
reserved area is now available (and there's a comment below that to that
effect, also), so the .sme section would basically be discarded and
re-claimed for general page usage.

> 
> How much does compressing 8MB of zeros slow down my kernel compile? ;)

Heh, I didn't measure that. :)

Thanks,
Tom

>