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Date: Wed, 29 Nov 2017 20:08:31 +0300
From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Borislav Petkov <bp@suse.de>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Ingo Molnar <mingo@redhat.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Cyrill Gorcunov <gorcunov@openvz.org>, Andi Kleen <ak@linux.intel.com>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCHv2 0/4] x86: 5-level related changes into decompression
 code
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On Wed, Nov 29, 2017 at 05:40:32PM +0100, Thomas Gleixner wrote:
> On Wed, 29 Nov 2017, Kirill A. Shutemov wrote:
> 
> > 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.
> 
> It needs to be fixed now. Because that problem exists in 4.14

Okay.

We're really early in the boot -- startup_64 in decompression code -- and
I don't know a way print a message there. Is there a way?

no_longmode handled by just hanging the machine. Is it enough for no_la57
case too?

-- 
 Kirill A. Shutemov