Re: [Xen-devel] debian stretch dom0 + xen 4.9 fails to boot

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Juergen Gross [mailto:jgross@xxxxxxxx]
> Sent: 07 June 2017 12:57
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>; Andrew Cooper
> <Andrew.Cooper3@xxxxxxxxxx>; Jan Beulich <JBeulich@xxxxxxxx>
> Cc: xen-devel (xen-devel@xxxxxxxxxxxxxxxxxxxx) <xen-
> devel@xxxxxxxxxxxxxxxxxxxx>; Julien Grall (julien.grall@xxxxxxx)
> <julien.grall@xxxxxxx>; 'Boris Ostrovsky' <boris.ostrovsky@xxxxxxxxxx>
> Subject: Re: [Xen-devel] debian stretch dom0 + xen 4.9 fails to boot
> 
> On 07/06/17 13:06, Paul Durrant wrote:
> >> -----Original Message-----
> >> From: Xen-devel [mailto:xen-devel-bounces@xxxxxxxxxxxxx] On Behalf Of
> >> Paul Durrant
> >> Sent: 07 June 2017 11:37
> >> To: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; 'Juergen Gross'
> >> <jgross@xxxxxxxx>; Jan Beulich <JBeulich@xxxxxxxx>
> >> Cc: xen-devel (xen-devel@xxxxxxxxxxxxxxxxxxxx) <xen-
> >> devel@xxxxxxxxxxxxxxxxxxxx>; Julien Grall (julien.grall@xxxxxxx)
> >> <julien.grall@xxxxxxx>; 'Boris Ostrovsky' <boris.ostrovsky@xxxxxxxxxx>
> >> Subject: Re: [Xen-devel] debian stretch dom0 + xen 4.9 fails to boot
> >>
> >>> -----Original Message-----
> >> [snip]
> >>>>>
> >>>>> TBH: I really can't see what is wrong with that patch. The only change
> >>>>> which should be able to break something seems to be the reduction
> of
> >>> the
> >>>>> wakeup stack size to 3kB, but this shouldn't affect booting the system
> >>>>> at all...
> >>>>>
> >>>> Yeah, my next test is going to be increasing the size of the wakeup
> stack
> >>> again, but there is really nothing obviously wrong with the patch.
> >>>
> >>> My gut feeling is that there is some path through boot (tickled by these
> >>> two machines) which is clobbering the wrong piece of memory, which
> was
> >>> previously safe and is now not, because of the rearrangements here.
> >>>
> >>> Debugging these machines is very tricky, because they have no serial or
> >>> IMPI whatsoever.
> >>>
> >>
> >> It does appear to be a layout issue. If I modify the code to just set
> >> wakeup_stack to wakeup_stack_start + PAGE_SIZE, so it has the full 4k
> then I
> >> still get the problem. However if I then move that code block that includes
> >> wakeup.S and move it to the end of trampoline.S so that wakup code and
> >> stack are once again located at the end then the problem goes away.
> >>
> >
> > It appears that it is just the code that needs to go at the end. The 
> > following
> patch is sufficient to avoid the problem. This may be preferable to a full
> reversion...
> 
> I believe this is wrong. You risk the wakeup_stack extending into wakeup
> code and the main reason of the patch is gone, as now the permanent
> trampoline no longer is on a single page.
> 

I must be misunderstanding something then. The stack grows down from 
wakeup_stack towards wakeup_stack_start doesn't it? So why would there be an 
issue with the stack overwriting wakeup code?

  Paul

> 
> Juergen
> 
> >
> >   Paul
> >
> > diff --git a/xen/arch/x86/boot/trampoline.S
> b/xen/arch/x86/boot/trampoline.S
> > index 4d640f3fcd..7709a782f9 100644
> > --- a/xen/arch/x86/boot/trampoline.S
> > +++ b/xen/arch/x86/boot/trampoline.S
> > @@ -156,7 +156,7 @@ start64:
> >          movabs  $__high_start,%rax
> >          jmpq    *%rax
> >
> > -#include "wakeup.S"
> > +ENTRY(wakeup_stack_start)
> >
> >  /* The first page of trampoline is permanent, the rest boot-time only. */
> >  /* Reuse the boot trampoline on the 1st trampoline page as stack for
> wakeup. */
> > @@ -280,3 +280,4 @@ rm_idt: .word   256*4-1, 0, 0
> >  #include "mem.S"
> >  #include "edd.S"
> >  #include "video.S"
> > +#include "wakeup.S"
> > diff --git a/xen/arch/x86/boot/wakeup.S b/xen/arch/x86/boot/wakeup.S
> > index f9632eef95..d4824b55d5 100644
> > --- a/xen/arch/x86/boot/wakeup.S
> > +++ b/xen/arch/x86/boot/wakeup.S
> > @@ -173,5 +173,3 @@ bogus_saved_magic:
> >          movw    $0x0e00 + 'S', 0xb8014
> >          jmp     bogus_saved_magic
> >
> > -/* Stack for wakeup: rest of first trampoline page. */
> > -ENTRY(wakeup_stack_start)
> >
> >>   Paul
> >>
> >> _______________________________________________
> >> Xen-devel mailing list
> >> Xen-devel@xxxxxxxxxxxxx
> >> https://lists.xen.org/xen-devel

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