Re: debian stretch dom0 + xen 4.9 fails to boot

[Paul Durrant <Paul.Durrant@citrix.com>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@suse.com]
> Sent: 12 June 2017 14:55
> To: Paul Durrant <Paul.Durrant@citrix.com>
> Cc: Julien Grall (julien.grall@arm.com) <julien.grall@arm.com>; Andrew
> Cooper <Andrew.Cooper3@citrix.com>; xen-devel(xen-
> devel@lists.xenproject.org) <xen-devel@lists.xenproject.org>; 'Boris
> Ostrovsky' <boris.ostrovsky@oracle.com>; Juergen Gross
> <jgross@suse.com>
> Subject: RE: [Xen-devel] debian stretch dom0 + xen 4.9 fails to boot
> 
> >>> On 12.06.17 at 14:05, <Paul.Durrant@citrix.com> wrote:
> >>  -----Original Message-----
> >> From: Jan Beulich [mailto:JBeulich@suse.com]
> >> Sent: 12 June 2017 12:12
> >> To: Paul Durrant <Paul.Durrant@citrix.com>
> >> Cc: Julien Grall (julien.grall@arm.com) <julien.grall@arm.com>; Andrew
> >> Cooper <Andrew.Cooper3@citrix.com>; xen-devel(xen-
> >> devel@lists.xenproject.org) <xen-devel@lists.xenproject.org>; 'Boris
> >> Ostrovsky' <boris.ostrovsky@oracle.com>; Juergen Gross
> >> <jgross@suse.com>
> >> Subject: RE: [Xen-devel] debian stretch dom0 + xen 4.9 fails to boot
> >>
> >> >>> On 12.06.17 at 12:53, <Paul.Durrant@citrix.com> wrote:
> >> >>  -----Original Message-----
> >> > [snip]
> >> >> > >
> >> >> > > What do you think it best to do for Xen 4.9? Hardcoding a 4k
> alignment
> >> is
> >> >> > > clearly easy and would work around this BIOS issue but, as you say,
> it
> >> >> does
> >> >> > > grow the image. Reverting Juergen's patch also works round the
> issue,
> >> >> but
> >> >> > > that is more by luck. Re-working the code is preferable, but I guess
> it's
> >> >> too
> >> >> > > late to introduce such code-churn in 4.9.
> >> >> >
> >> >> > Reverting Jürgen's code is out of question with all the information
> >> >> > you've gathered by now. I think re-working the EDD code slightly
> >> >> > is the best option. Would you mind giving the attached patch a
> >> >> > try? This still slightly grows the trampoline due to a few more
> >> >> > instructions being needed, but should still be far better than
> >> >> > embedding a whole 4k buffer (and then later finding a BIOS/disk
> >> >> > combination which wants even more). Note that I've left a tiny
> >> >> > bit of debugging code in there.
> >> >> >
> >> >>
> >> >> Sure, I'll give that a go now.
> >> >>
> >> >
> >> > That worked fine:
> >> >
> >> > (XEN) MBR[80] @ 85e0 (86000)
> >>
> >> But that's contrary to your earlier findings: Didn't you say simply
> >> avoiding a 4k-boundary wasn't enough? And it certainly tells us
> >> that this isn't a 4k drive (or at least the BIOS doesn't surface 4k
> >> sectors) - I was really expecting a larger gap between the two
> >> logged values.
> >>
> >
> > I'll go dump out the edd and double check what it is saying.
> >
> > My findings indicated that the problem seemed to be doing a read that
> > spanned a 4k boundary caused a problem, so using 0x85e00 would be safe.
> The
> > anomaly was that simply aligning the edd_info buffer and a 512 byte
> boundary
> > and continuing to use that for reading did not work.
> 
> But a 512-byte aligned 512-byte buffer can't possibly cross a page
> boundary.

Indeed, which is why I was perplexed. I found that 0x60e00 was ok. Your patch chose 0x85e00, which was ok too, but for some reason a '.align 512' in front of boot_edd_info yielded an address which was not ok. I just checked what address that yielded though (by booting with edd=off to avoid the hang) and it was 0x86f40... which clearly means that '.align 512' is not doing what I thought it would do.

  Paul

> 
> >> > so you can add my Tested-by to that.
> >>
> >> I.e. I'm not sure about this, as I'm still uncertain whether some
> >> corruption didn't again occur. Of course APs coming up properly
> >> would already be a relatively good sign (as now the permanent
> >> part of the trampoline would be the predestined area for
> >> corruption to occur in).
> >>
> >
> > None of my findings ever indicated memory corruption (although there, of
> > course, may have been some that I happened to miss), but rather
> misbehaviour
> > of the int13 handler itself - either locking up, having odd effects (e.g.
> > black screen), or both.
> 
> Ah, I didn't understand it this way so far, and instead had implied
> that the handler did return, but corrupt our trampoline area in
> one way or another.
> 
> Jan
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