Re: [Xen-devel] Linux 4.19.5 fails to boot as Xen dom0

["Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>]

On Thu, Nov 29, 2018 at 02:24:47PM +0000, Kirill A. Shutemov wrote:
> On Thu, Nov 29, 2018 at 01:35:17PM +0000, Juergen Gross wrote:
> > On 29/11/2018 14:26, Kirill A. Shutemov wrote:
> > > On Thu, Nov 29, 2018 at 09:41:25AM +0000, Juergen Gross wrote:
> > >> On 29/11/2018 02:22, Hans van Kranenburg wrote:
> > >>> Hi,
> > >>>
> > >>> As also seen at:
> > >>> https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=914951
> > >>>
> > >>> Attached there are two serial console output logs. One is starting with
> > >>> Xen 4.11 (from debian unstable) as dom0, and the other one without Xen.
> > >>>
> > >>> [    2.085543] BUG: unable to handle kernel paging request at
> > >>> ffff888d9fffc000
> > >>> [    2.085610] PGD 200c067 P4D 200c067 PUD 0
> > >>> [    2.085674] Oops: 0000 [#1] SMP NOPTI
> > >>> [    2.085736] CPU: 1 PID: 1 Comm: swapper/0 Not tainted
> > >>> 4.19.0-trunk-amd64 #1 Debian 4.19.5-1~exp1+pvh1
> > >>> [    2.085823] Hardware name: HP ProLiant DL360 G7, BIOS P68 05/21/2018
> > >>> [    2.085895] RIP: e030:ptdump_walk_pgd_level_core+0x1fd/0x490
> > >>> [...]
> > >>
> > >> The offending stable commit is 4074ca7d8a1832921c865d250bbd08f3441b3657
> > >> ("x86/mm: Move LDT remap out of KASLR region on 5-level paging"), this
> > >> is commit d52888aa2753e3063a9d3a0c9f72f94aa9809c15 upstream.
> > >>
> > >> Current upstream kernel is booting fine under Xen, so in general the
> > >> patch should be fine. Using an upstream kernel built from above commit
> > >> (with the then needed Xen fixup patch 1457d8cf7664f34c4ba534) is fine,
> > >> too.
> > >>
> > >> Kirill, are you aware of any prerequisite patch from 4.20 which could be
> > >> missing in 4.19.5?
> > > 
> > > I'm not.
> > > 
> > > Let me look into this.
> > > 
> > 
> > What is making me suspicious is the failure happening just after
> > releasing the init memory. Maybe there is an access to .init.data
> > segment or similar? The native kernel booting could be related to the
> > usage of 2M mappings not being available in a PV-domain.
> 
> Ahh.. Could you test this:
> 
> diff --git a/arch/x86/mm/dump_pagetables.c b/arch/x86/mm/dump_pagetables.c
> index a12afff146d1..7dec63ec7aab 100644
> --- a/arch/x86/mm/dump_pagetables.c
> +++ b/arch/x86/mm/dump_pagetables.c
> @@ -496,7 +496,7 @@ static inline bool is_hypervisor_range(int idx)
>        * ffff800000000000 - ffff87ffffffffff is reserved for
>        * the hypervisor.
>        */
> -     return  (idx >= pgd_index(__PAGE_OFFSET) - 16) &&
> +     return  (idx >= pgd_index(__PAGE_OFFSET) - 17) &&
>               (idx <  pgd_index(__PAGE_OFFSET));
>  #else
>       return false;

Or, better, this:

diff --git a/arch/x86/mm/dump_pagetables.c b/arch/x86/mm/dump_pagetables.c
index a12afff146d1..8c04fadc4423 100644
--- a/arch/x86/mm/dump_pagetables.c
+++ b/arch/x86/mm/dump_pagetables.c
@@ -496,8 +496,8 @@ static inline bool is_hypervisor_range(int idx)
         * ffff800000000000 - ffff87ffffffffff is reserved for
         * the hypervisor.
         */
-       return  (idx >= pgd_index(__PAGE_OFFSET) - 16) &&
-               (idx <  pgd_index(__PAGE_OFFSET));
+       return  (idx >= pgd_index(LDT_BASE_ADDR) - 16) &&
+               (idx <  pgd_index(LDT_BASE_ADDR));
 #else
        return false;
 #endif
diff --git a/arch/x86/xen/mmu_pv.c b/arch/x86/xen/mmu_pv.c
index 2c84c6ad8b50..b078a5b0ac91 100644
--- a/arch/x86/xen/mmu_pv.c
+++ b/arch/x86/xen/mmu_pv.c
@@ -652,7 +652,7 @@ static int __xen_pgd_walk(struct mm_struct *mm, pgd_t *pgd,
         * will end up making a zero-sized hole and so is a no-op.
         */
        hole_low = pgd_index(USER_LIMIT);
-       hole_high = pgd_index(PAGE_OFFSET);
+       hole_high = pgd_index(LDT_BASE_ADDR);
 
        nr = pgd_index(limit) + 1;
        for (i = 0; i < nr; i++) {
-- 
 Kirill A. Shutemov

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