Re: [Xen-devel] Xen BUG at page_alloc.c:1738 (Xen 4.5)

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 29/05/15 12:17, M A Young wrote:
>
>>> I did a bit of testing - xen-4.5.1-rc1 built on Fedora 22 (gcc5) doesn't 
>>> boot for me, but if I replace xen.gz with one from the same code built on 
>>> Fedora 21 (gcc4) then it does boot. There are rpms and build logs 
>>> available via 
>>> http://copr.fedoraproject.org/coprs/myoung/xentest/build/93366/
>>> if anyone else wants to do some testing.
>>>
>>>     Michael Young
>> Do you have easy access to xen-syms from each build?
> Yes.
>

Thankyou very much.

GCC 5 is indeed miscompiling the code. Comparing the fc21 vs fc22 builds:

The C snippet from mmio_ro_do_page_fault():

struct page_info *page = mfn_to_page(mfn);
struct domain *owner = page_get_owner_and_reference(page);
if ( owner )
    put_page(page);

In fc21 is:

movabs $0xffff82e000000000,%rbp
shr    %cl,%rax
or     %rdx,%rax
shl    $0x5,%rax
add    %rax,%rbp
mov    %rbp,%rdi
callq  ffff82d080186900 <page_get_owner_and_reference>
test   %rax,%rax
mov    %rax,%r12
je     ffff82d080189c4e <mmio_ro_do_page_fault+0x11e>
mov    %rbp,%rdi
callq  ffff82d080188ec0 <put_page>

and in fc22 is:

movabs $0xffff82e000000000,%r8
shr    %cl,%rax
or     %rdx,%rax
shl    $0x5,%rax
lea    (%r8,%rax,1),%rdi
callq  ffff82d0801874f0 <page_get_owner_and_reference>
test   %rax,%rax
mov    %rax,%rbp
je     ffff82d08018ca14 <mmio_ro_do_page_fault+0x114>
mov    %r8,%rdi
callq  ffff82d080189a90 <put_page>

"lea (%r8,%rax,1),%rdi" in FC22 is slightly shorter than "add %rax,%rbp;
mov %rbp,%rdi" in FC21.  In both cases %rdi is now 'page' from the C
snippet.

In FC21, the result is stored in %rbp, then reloaded from %rbp into %rdi
for call to put_page().

However, in FC22, the result of the calculation is only held in %rdi,
and clobbered by the call to page_get_owner_and_reference().  When it
comes to call put_page(), %r8 is reloaded, which is still a pointer to
the base of the frametable, not the page we actually took a reference on.

FC22 is miscompiling the C to:

struct page_info *page = mfn_to_page(mfn);
struct domain *owner = page_get_owner_and_reference(page);
if ( owner )
    put_page(mfn_to_page(0));

which is wrong, and why free_domheap_pages() does legitimately complain
about the wonky refcount.

~Andrew

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