linux-kernel - Re: [GIT PULL] Small Xen bugfixes

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Message-ID: <4CCB1E71.3060600@goop.org>
Date:	Fri, 29 Oct 2010 12:20:17 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	"Xen-devel@...ts.xensource.com" <Xen-devel@...ts.xensource.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ian Campbell <Ian.Campbell@...rix.com>,
	Vasiliy G Tolstov <v.tolstov@...fip.ru>
Subject: Re: [GIT PULL] Small Xen bugfixes

 On 10/29/2010 12:08 PM, Linus Torvalds wrote:
> On Fri, Oct 29, 2010 at 11:57 AM, Jeremy Fitzhardinge <jeremy@...p.org> wrote:
>>    * fix dom0 boot on systems whose E820 doesn't completely cover the
>>      ISA address space.  This fixes a crash on a Dell R310.
> Hmm. This clashes with my current tree.

Bugger, so it does.  I just did a test merge with no complaint though;
what happened?

I'll redo the patch anyway to fix the below.

> And that conflict is trivial to fix up, but the thing is, I think the
> patch that comes from your tree is worse than what is already there.
>
> Why is that simple unconditional
>
>     e820_add_region(ISA_START_ADDRESS, ISA_END_ADDRESS - ISA_START_ADDRESS,
>            E820_RESERVED);
>
> not just always the right thing? Why do you have a separate hack for
> dom0 in xen_release_chunk() instead? That just looks bogus.

Yes, we actually had this discussion.  I was for making the
e820_add_region unconditional, and Ian's counter was that it could be
done in the common code rather than Xen-specific.

> The normal logic we use on PC's is to just always reserve the low 64kB
> of memory, and the whole ISA space. Why doesn't Xen just do the same?

The specific issue is that the Xen domain returns any memory that's not
covered by an E820 entry back to Xen, mostly to make sure that memory
isn't wasted by being shadowed by PCI devices.  But it was also doing
this in the sub-1M region, which on all the machines I've tested on is
completely covered.  But on a Dell R310 there's a little 2-page gap
where some ACPI stuff is lurking, that was being released back to Xen so
it couldn't be accessed from Linux any more.

The fix is to just make sure the whole low region is covered (or at
least the 640k-1M space).

I'll rework the patch.

Thanks,
    J

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