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Re: [Xen-devel] [BUG 1747]Guest could't find bootable device with memory more than 3600M

On 13/06/13 16:16, Ian Campbell wrote:
On Thu, 2013-06-13 at 14:54 +0100, George Dunlap wrote:
On 13/06/13 14:44, Stefano Stabellini wrote:
On Wed, 12 Jun 2013, George Dunlap wrote:
On 12/06/13 08:25, Jan Beulich wrote:
On 11.06.13 at 19:26, Stefano Stabellini
<stefano.stabellini@xxxxxxxxxxxxx> wrote:
I went through the code that maps the PCI MMIO regions in hvmloader
(tools/firmware/hvmloader/pci.c:pci_setup) and it looks like it already
maps the PCI region to high memory if the PCI bar is 64-bit and the MMIO
region is larger than 512MB.

Maybe we could just relax this condition and map the device memory to
high memory no matter the size of the MMIO region if the PCI bar is
I can only recommend not to: For one, guests not using PAE or
PSE-36 can't map such space at all (and older OSes may not
properly deal with 64-bit BARs at all). And then one would generally
expect this allocation to be done top down (to minimize risk of
running into RAM), and doing so is going to present further risks of
incompatibilities with guest OSes (Linux for example learned only in
2.6.36 that PFNs in ioremap() can exceed 32 bits, but even in
3.10-rc5 ioremap_pte_range(), while using "u64 pfn", passes the
PFN to pfn_pte(), the respective parameter of which is
"unsigned long").

I think this ought to be done in an iterative process - if all MMIO
regions together don't fit below 4G, the biggest one should be
moved up beyond 4G first, followed by the next to biggest one
First of all, the proposal to move the PCI BAR up to the 64-bit range is a
temporary work-around.  It should only be done if a device doesn't fit in the
current MMIO range.

We have three options here:
1. Don't do anything
2. Have hvmloader move PCI devices up to the 64-bit MMIO hole if they don't
3. Convince qemu to allow MMIO regions to mask memory (or what it thinks is
4. Add a mechanism to tell qemu that memory is being relocated.

Number 4 is definitely the right answer long-term, but we just don't have time
to do that before the 4.3 release.  We're not sure yet if #3 is possible; even
if it is, it may have unpredictable knock-on effects.

Doing #2, it is true that many guests will be unable to access the device
because of 32-bit limitations.  However, in #1, *no* guests will be able to
access the device.  At least in #2, *many* guests will be able to do so.  In
any case, apparently #2 is what KVM does, so having the limitation on guests
is not without precedent.  It's also likely to be a somewhat tested
configuration (unlike #3, for example).
I would avoid #3, because I don't think is a good idea to rely on that
I would also avoid #4, because having seen QEMU's code, it's wouldn't be
easy and certainly not doable in time for 4.3.

So we are left to play with the PCI MMIO region size and location in

I agree with Jan that we shouldn't relocate unconditionally all the
devices to the region above 4G. I meant to say that we should relocate
only the ones that don't fit. And we shouldn't try to dynamically
increase the PCI hole below 4G because clearly that doesn't work.
However we could still increase the size of the PCI hole below 4G by
default from start at 0xf0000000 to starting at 0xe0000000.
Why do we know that is safe? Because in the current configuration
hvmloader *already* increases the PCI hole size by decreasing the start
address every time a device doesn't fit.
So it's already common for hvmloader to set pci_mem_start to
0xe0000000, you just need to assign a device with a PCI hole size big
Isn't this the exact case which is broken? And therefore not known safe
at all?

My proposed solution is:

- set 0xe0000000 as the default PCI hole start for everybody, including
What is the impact on existing qemu-trad guests?

It does mean that guest which were installed with a bit less than 4GB
RAM may now find a little bit of RAM moves above 4GB to make room for
the bigger whole. If they can dynamically enable PAE that might be ok.

Does this have any impact on Windows activation?

- move above 4G everything that doesn't fit and support 64-bit bars
- print an error if the device doesn't fit and doesn't support 64-bit
Also, as I understand it, at the moment:
1. Some operating systems (32-bit XP) won't be able to use relocated devices
2. Some devices (without 64-bit BARs) can't be relocated
3. qemu-traditional is fine with a resized <4GiB MMIO hole.

So if we have #1 or #2, at the moment an option for a work-around is to
use qemu-traditional.

However, if we add your "print an error if the device doesn't fit", then
this option will go away -- this will be a regression in functionality
from 4.2.
Only if print an error also involves aborting. It could print an error
(lets call it a warning) and continue, which would leave the workaround

No, because if hvmloader doesn't increase the size of the MMIO hole, then the device won't actually work. The guest will boot, but the OS will not be able to use it.


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