linux-kernel - Re: [PATCHv2 0/4] x86: 5-level related changes into decompression code

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Date:   Wed, 29 Nov 2017 19:13:49 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Borislav Petkov <bp@...e.de>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...capital.net>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        Andi Kleen <ak@...ux.intel.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2 0/4] x86: 5-level related changes into decompression
 code

On Wed, Nov 29, 2017 at 04:49:08PM +0100, Borislav Petkov wrote:
> On Sat, Nov 11, 2017 at 01:06:41AM +0300, Kirill A. Shutemov wrote:
> > Hi Ingo,
> > 
> > Here's updated changes that prepare the code to boot-time switching between
> > paging modes and handle booting in 5-level mode when bootloader put kernel
> > image above 4G, but haven't enabled 5-level paging for us.
> 
> Btw, if I enable CONFIG_X86_5LEVEL with 4.15-rc1 on an AMD box, the box
> triple-faults and ends up spinning in a reboot loop. Even though it
> should say:
> 
> early console in setup code
> This kernel requires the following features not present on the CPU:
> la57 
> Unable to boot - please use a kernel appropriate for your CPU.
> 
> and halt.
> 
> A kvm guest still does that but baremetal triple-faults.
> 
> Ideas?

Looks like we call check_cpuflags() too late. 5-level paging gets enabled
before image decompression started.

For qemu/kvm it works because it's supported in softmmu, even if not
advertised in cpuid.

I'm not sure if it worth fixing on its own. I would rather get boot-time
switching code upstream sooner. It will get problem go away naturally.

-- 
 Kirill A. Shutemov