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

From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Borislav Petkov <bp@suse.de>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Ingo Molnar <mingo@redhat.com>,
	x86@kernel.org, Thomas Gleixner <tglx@linutronix.de>,
	"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
Date: Wed, 29 Nov 2017 19:13:49 +0300	[thread overview]
Message-ID: <20171129161349.d7ksuhwhdamloty6@node.shutemov.name> (raw)
In-Reply-To: <20171129154908.6y4st6xc7hbsey2v@pd.tnic>

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