Re: [Qemu-devel] Bug with TARGET_PHYS_ADDR_SPACE_BITS

[Anthony Liguori]

Chris Lalancette wrote:
> 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?
>   

Right now, the code just can't handle TARGET_PHYS_ADDR_SPACES_BITS > 
32.  This may help you:

diff --git a/exec.c b/exec.c
index a0aa7dc..cac7a87 100644
--- a/exec.c
+++ b/exec.c
@@ -153,7 +153,7 @@ typedef struct PhysPageDesc {
 */
#define L1_BITS (TARGET_VIRT_ADDR_SPACE_BITS - L2_BITS - TARGET_PAGE_BITS)
#else
-#define L1_BITS (32 - L2_BITS - TARGET_PAGE_BITS)
+#define L1_BITS (TARGET_PHYS_ADDR_SPACE_BITS - L2_BITS - TARGET_PAGE_BITS)
#endif

#define L1_SIZE (1 << L1_BITS)

But there are a lot more places in the code that assume 
TARGET_PHYS_ADDR_SPACE_BITS == 32.  I'm inclined to think that we should 
change it back to 32.

Regards,

Anthony Liguori

> Thanks,
> Chris Lalancette
>
>
>