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

[Thomas Gleixner]

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

Thanks,

	tglx