Re: 2.6.25-rc1 xen pvops regression

Ian Campbell
2008-02-20T08:52:21Z

On Tue, 2008-02-19 at 23:43 -0800, H. Peter Anvin wrote:
> Ian Campbell wrote:
> > On Mon, 2008-02-18 at 02:40 -0800, Joel Becker wrote:
> >> On Sun, Feb 17, 2008 at 06:49:21PM +0000, Ian Campbell wrote:
> >
> >>> x86/xen: Do not scan for DMI unless the DMI region is reserved by e820.
> >
> >> This fixed it. I'm now booting successfully. Thank you!
> >
> > Excellent. Jeremy, are you happy for this to go in?
> >
>
> NAK!
>
> It's pretty standard for 0xf0000...0x100000 to be marked RESERVED in
> E820 on real hardware (including the system I'm typing on right now.)
> It is so marked to indicate that hardware cannot be mapped into that
> space. However, you can't rely on this fact -- heck, you can't rely on
> E820 even existing on a real machine. I have specimens of real-life
> machines that go both ways.
>
> This patch WILL break real hardware.
>
> What's particularly damning is that it's titled "x86/xen: Do not scan
> for DMI unless the DMI region is reserved by e820." whereas in fact it
> changes (breaks) generic code.

Sorry, I was trying to indicate that it was a generic change which was
motivated by Xen support, but you're right it did look like I was saying
it was a Xen only change.

As far as the actual change goes I was assuming that any machine that
has DMI/SMBIOS would easily be new enough to have an E820 which could be
expected to reserve this region. Looks like I was mistaken about how
long E820 had been around and/or how reliably it is used to reserve the
tables.

Anyway, will have to think of another solution.

Ian.

--
Ian Campbell

Just because the message may never be received does not mean it is
not worth sending.