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

[Juergen Gross <jgross@xxxxxxxx>]

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.


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
>>
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