[Xen-devel] [Q] PV page table seems to be reset to zero.. what happened?

[Min Lee <min.lee@xxxxxxxxxx>]

Hello. I'm having some trouble,
I'm working on page table to basically build reverse mapping..
So I'm intercepting mapping changes in mod_l1_entry() and 
[alloc,free]_l1_table() and it seemed to work for some page tables.
However, my L1 page table values are changed somewhere that I don't 
know.. this happens to do_update_va_mapping().
Here's the situation..
(1) do_update_va_mapping() calls mod_l1_entry() and pte's value is set.
(2) later do_update_va_mapping() find this pte value zero!!

see the below log from do_update_va_mapping() and mod_l1_entry(),
b7d6 is mfn, and dcc is offset in this page table.
'=0' means its original value is 0.
so it changed it to 'f79e067'..
(virtual address from the guset is b7f731d4)

(XEN) (va:b7d6:dcc(dcc)=0)
(XEN) 1@1 t:b7d6,dcc O:0(f79e067) N:f79e067 a:b7f731d4 r:a
.
.
.
(XEN) (va:b7d6:dcc(dcc)=0)
(XEN) 1@1 t:b7d6,dcc O:0(f79e065) N:f79e065 a:b7f73000 r:a

afterwards, it again calls do_update_va_mapping(), but now it reports 
its old value as 0, not 'f79e067' and try to change its value to 
f79e065. (it means, it's just changing its flags only)..
I was expecting this reports that the original value is f79e067, as it 
was set before.
This might seem to be racing, but note that they're same VCPU@PCPU (1@1) 
and there was rather long time between these two event. so I doubt it's 
racing problem. Also it happens pretty regularly.
It just seems to me someone is resetting/changing page table value to 
zero.. anybody can give me some hints/or comment?
Where else these mapping can change? I'm working on IA32, no pae,.
thanks in advance.

Min

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