linux-kernel - Re: [PATCHv2 08/10] x86/mm: Replace compile-time checks for 5-level with runtime-time

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Date:   Wed, 2 Aug 2017 09:44:54 +0200
From:   Juergen Gross <jgross@...e.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...capital.net>,
        Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2 08/10] x86/mm: Replace compile-time checks for 5-level
 with runtime-time

On 01/08/17 21:11, Kirill A. Shutemov wrote:
> On Tue, Aug 01, 2017 at 07:14:57PM +0200, Juergen Gross wrote:
>> On 01/08/17 16:44, Kirill A. Shutemov wrote:
>>> On Tue, Aug 01, 2017 at 09:46:56AM +0200, Juergen Gross wrote:
>>>> On 26/07/17 18:43, Kirill A. Shutemov wrote:
>>>>> On Wed, Jul 26, 2017 at 09:28:16AM +0200, Juergen Gross wrote:
>>>>>> On 25/07/17 11:05, Kirill A. Shutemov wrote:
>>>>>>> On Tue, Jul 18, 2017 at 04:24:06PM +0200, Juergen Gross wrote:
>>>>>>>> Xen PV guests will never run with 5-level-paging enabled. So I guess you
>>>>>>>> can drop the complete if (IS_ENABLED(CONFIG_X86_5LEVEL)) {} block.
>>>>>>>
>>>>>>> There is more code to drop from mmu_pv.c.
>>>>>>>
>>>>>>> But while there, I thought if with boot-time 5-level paging switching we
>>>>>>> can allow kernel to compile with XEN_PV and XEN_PVH, so the kernel image
>>>>>>> can be used in these XEN modes with 4-level paging.
>>>>>>>
>>>>>>> Could you check if with the patch below we can boot in XEN_PV and XEN_PVH
>>>>>>> modes?
>>>>>>
>>>>>> We can't. I have used your branch:
>>>>>>
>>>>>> git://git.kernel.org/pub/scm/linux/kernel/git/kas/linux.git
>>>>>> la57/boot-switching/v2
>>>>>>
>>>>>> with this patch applied on top.
>>>>>>
>>>>>> Doesn't boot PV guest with X86_5LEVEL configured (very early crash).
>>>>>
>>>>> Hm. Okay.
>>>>>
>>>>> Have you tried PVH?
>>>>>
>>>>>> Doesn't build with X86_5LEVEL not configured:
>>>>>>
>>>>>>   AS      arch/x86/kernel/head_64.o
>>>>>
>>>>> I've fixed the patch and split the patch into two parts: cleanup and
>>>>> re-enabling XEN_PV and XEN_PVH for X86_5LEVEL.
>>>>>
>>>>> There's chance that I screw somthing up in clenaup part. Could you check
>>>>> that?
>>>>
>>>> Build is working with and without X86_5LEVEL configured.
>>>>
>>>> PV domU boots without X86_5LEVEL configured.
>>>>
>>>> PV domU crashes with X86_5LEVEL configured:
>>>>
>>>> xen_start_kernel()
>>>>   x86_64_start_reservations()
>>>>     start_kernel()
>>>>       setup_arch()
>>>>         early_ioremap_init()
>>>>           early_ioremap_pmd()
>>>>
>>>> In early_ioremap_pmd() there seems to be a call to p4d_val() which is an
>>>> uninitialized paravirt operation in the Xen pv case.
>>>
>>> Thanks for testing.
>>>
>>> Could you check if patch below makes a difference?
>>
>> A little bit better. I get a panic message with backtrace now:
> 
> Are you running with 512m of ram or so?

Yes. :-)

> There's known issue with sparse mem: it still allocate data structures as
> if there's 52-bit phys address space even for p4d_folded case.
> 
> I'm looking this.
> 
> Try to bump memory size to 2g or so for now.

That did the trick!

PV domU is coming up now with a 5-level paging enabled kernel.


Juergen