[Qemu-devel] Bug with TARGET_PHYS_ADDR_SPACE_BITS

[Chris Lalancette <clalance@redhat.com>]

Hello,
     oVirt is currently using straight x86_64 qemu emulation for certain parts
of the architecture (we mostly use KVM, but need to use full emulation for a
couple of parts).  We recently upgraded our userspace package to kvm-72, but
found that we could not PXE boot guests when we were doing full emulation (under
kvm, we could PXE boot just fine).  We also tried using qemu SVN tip, with
similar results.  We ended up doing a bisect, and tracked down the problem to
this commit (from the kvm repo, but pulled from qemu):

http://git.kernel.org/?p=linux/kernel/git/amit/kvm-userspace.git;a=commit;h=468f7507339a5236bff8ab339eb0c1b019a95fda

The important changes in there in terms of this bug revolves around
TARGET_PHYS_ADDR_SPACE_BITS in exec.c.  If I change that back to 32 (what it was
before this patch for x86_64), the PXE boot succeeds.  Also, if I remove
TARGET_PHYS_ADDR_SPACE_BITS > 32 conditional code in phys_page_find_alloc(), but
leave TARGET_PHYS_ADDR_SPACE_BITS as 42, the PXE boot also works.  I can't claim
to understand the conditional code I've compiled out, so I'm not sure where the
bug would be.  Does anyone have an idea what the problem might be?

Thanks,
Chris Lalancette