Red Hat Bugzilla – Bug 425794
PCI allocation errors result in Xserver resetting the machine
Last modified: 2009-01-09 00:30:58 EST
On an ASUS G1S laptop, w/ 3GB ram, the stock kernel will reallocate the
video card BAR to 0x1_0000_0000 - 0x1_0fff_ffff. The Xserver does not
appear to be able to handle 64-bit PCI addresses, and the machine will
either hang (xv driver) or reset (nvidia driver).
The initial reason that the kernel reallocates the BAR in the first
place is that there is an rounding error bug in the x86-64 e820 driver.
The algorithm "rounds" 0xc000_0000 to the nearest 1M and comes up with
0xc004_0000. After this error, there is no longer a contiguous unallocated
256MB available below 4G, which leads to the BAR being placed at 4GB.
While investigating the problem, it occurred to me that there were other
potential PCI allocation problems on x86-64. For instance, there was
nothing preventing a true 32-bit BAR from being allocated above 4GB on
x86-64. After that is fixed, it is easy to add a quirk to work around
the Xserver bug by preventing VGA BAR allocation above 4GB.
One patch for each problem is attached.
Created attachment 289707 [details]
Fix for e820 rounding error
Created attachment 289708 [details]
Fix 32-bit BARs being allocated above 4GB
In addition, it adds an x86 quirk that forces VGA BARs to be allocated
within the low 4GB. Assuming that the Xserver were to be fixed, presumably
this restriction could be lifted. Especially since video card memory
sizes keep growing, there soon won't be room within the 1GB window left
for reports that similar problems affect other systems besides my G1S.
Can you post the complete dmesg and contents of /proc/iomem from the broken kernel?
Also, Linus has suggested an alternative for the second patch:
Created attachment 289932 [details]
Output from dmesg from 126.96.36.199-63.fc8
Created attachment 289933 [details]
Contents of /proc/iomem from 188.8.131.52-63.fc8
Created attachment 289934 [details]
Output from lspci -v
I wonder, what is pnp device 0:0b and why does it want 256MB of memory?
Also, looks like PnP wants to reserve all of system memory:
pnp: 00:0c: iomem range 0x100000-0xbfffffff could not be reserved
[ This is just cut-and-pasted from an email I sent. I don't think the
stupid rh bugzilla can interact with emails like sane people do, so
I'll just add this to the bugzilla history manually ]
That really is very unlucky. That 256M only goes at one point in the low
4GB, but the thing is, it fits perfectly well above it, and dammit, that
resource is explicitly a 64-bit resource or a really good reason.
However, I wonder about that
e0000000-efffffff : pnp 00:0b
thing. I actually suspect that that whole allocation is literally *meant*
for that 256MB graphics aperture, but the kernel explicitly avoids it
because it's listed in the PnP tables.
I wonder what the heck is the point of that pnp entry. Just for fun, can
you try to just disable CONFIG_PNP, and see if it all works then?
Björn Helgaas added to Cc to clarify what those pnp entries tend to mean,
and whether there is possibly some way to match up a specific pnp entry
with the PCI device that might want to use it. Because that is a nice
256MB region that really doesn't seem to make sense for anything else than
the graphics buffer - there's nothing else in your system that seems
likely (although I guess it could be for some docking port, but even then
I'd have expected one of the PCI bridges to map it!)
But apart from the question about that pnp 00:0b device, the kernel
resource allocation really does look perfectly fine, and while we could
shoe-horn it into the low 4GB in this case by just hoping that there is
nothing undocumented there (and there probably isn't), it's really
annoying considering that big graphics areas are a hell of a good reason
to use those 64-bit resources.
It's not like 256MB is even as large as they come, half-gig graphics cards
are getting to be fairly common at the high end, and X absolutely _has_ to
be able to handle a 64-bit address for those.
Also, I'm surprised it doesn't work with X already: the ChangeLog for X
says that there are "Minor fixes to the handling of 64-bit PCI BARs [..]"
in 184.108.40.206, so I'd have assumed that XFree86-4.7.0 should be able to
handle this perfectly well.
I'll add Keithp to the cc too, to see if the X issues can be clarified.
Maybe he can set us right. But maybe you just have an old X server? If so,
considering the situation, I really think the kernel has done a good job
already, and I'd be *very* nervous about making the kernel allocate new
PCI resources right after the end-of-memory thing.
I bet it would work in this case, but as mentioned, we definitely know of
cases where the BIOS did *not* document the magic memory region that was
stolen for UMA graphics, and trying to put PCI devices just after the top
of reserved memory in the e820 list causes machines to not work at all
because the address decoding will clash.
Of course, we could also make the minimum address more of a *hint*, and
only make the resource allocator only abut the top-of-known-memory when it
absolutely has to, but on the other hand, in this case it really doesn't
have to, since there's just _tons_ of space for 64-bit resources. So the
correct thing really does seem to be to just use the 64-bit hw that is
As posted on LKML, I suspect the reservation is for the MMCONFIG aperture and
therefore is valid and must be respected.
I don't think forcing VGA BARs to be allocated within the low 4GB is a good
approach. In some cases it may be impossible to allocate the BAR there, and it's
really an artificial restriction. The X server needs to be fixed to handle this
if it really can't right now.
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