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

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: 08 June 2017 14:19
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> Cc: Julien Grall (julien.grall@xxxxxxx) <julien.grall@xxxxxxx>; Andrew
> Cooper <Andrew.Cooper3@xxxxxxxxxx>; xen-devel(xen-
> devel@xxxxxxxxxxxxxxxxxxxx) <xen-devel@xxxxxxxxxxxxxxxxxxxx>;
> 'BorisOstrovsky' <boris.ostrovsky@xxxxxxxxxx>; Juergen Gross
> <jgross@xxxxxxxx>
> Subject: RE: [Xen-devel] debian stretch dom0 + xen 4.9 fails to boot
> 
> >>> On 08.06.17 at 14:42, <Paul.Durrant@xxxxxxxxxx> wrote:
> > For those following this...
> >
> > By poking characters at the screen and bisecting where they stopped, I
> have
> > narrowed the problem to the code in edd.S. I can successfully boot by
> setting
> > opt_edd=off on the Xen cmd line and I can also boot with the following
> patch
> > applied:
> >
> > diff --git a/xen/arch/x86/boot/edd.S b/xen/arch/x86/boot/edd.S
> > index 73371f98b5..5409f1d9a1 100644
> > --- a/xen/arch/x86/boot/edd.S
> > +++ b/xen/arch/x86/boot/edd.S
> > @@ -148,5 +148,6 @@ GLOBAL(boot_mbr_signature_nr)
> >          .byte   0
> >  GLOBAL(boot_mbr_signature)
> >          .fill   EDD_MBR_SIG_MAX*8,1,0
> > +       .align  4096
> >  GLOBAL(boot_edd_info)
> > -        .fill   512,1,0                         # big enough for a disc
> > sector
> > +        .fill   4096,1,0                         # big enough for a disc
> > sector
> >
> > (based on a hunch that the BIOS defaults to a 4K sector for my NVMe drive)
> >
> > I need to investigate some more but I do wonder whether the EDD info
> should
> > be read first to determine the appropriate size of memory buffer to use
> when
> > issuing the read of the MBR. Hardcoding a 4k reservation seems like the
> wrong
> > thing to do, even if it is sufficient for this BIOS.
> 
> boot_edd_info is being used for two things - reading the MBR of
> each disk and storing data retrieved from INT 13 Fn 41 and 48.
> The latter occupies 492 bytes (6 times 8+74). Which would make
> me guess the system has a 4k disk, and the BIOS doesn't abstract
> away this characteristic when handling INT 13 Fn 02 (which is
> supposed to only act in multiples of 512-byte sectors, as opposed
> to Fn 42).
> 
> The alternative of Fn 48 overflowing its buffer would seem less
> likely, especially with the buffer holding a size on input.

Yes, I tested with edd=skipmbr on the command line (and no patch applied) and 
the system booted, so it's definitely the MBR read that is at fault.

> 
> Do you, btw, really need both the size and alignment increases?
> 

At first I tried just increasing the .fill to 4096 but that did not seem to 
work. I have not found anything that says int13 0x2 buffers need to be 
aligned... but the BIOS being buggy in this respect I guess it could easily 
require that.
I'm just testing some more code to try to see exactly how much memory the MBR 
read scribbles on.

  Paul

> Jan


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