Re: Detecting deadlocks with hypervisor..

[Anthony Liguori <aliguori@us.ibm.com>]

Keir Fraser wrote:
>
> On 7 Apr 2006, at 18:11, T S wrote:
>
>> We took a look at the xc_linux_save() function ... and what we see is 
>> that
>> the canonicalize action is actually done by the Dom-0 (and not by the 
>> Dom-U);
>> Dom-0 is able to do this because it is able to access the page tables 
>> of Dom-U
>> as well as the pfn2mfn list of the Dom-U. Based on this, we think the 
>> Dom-0 can
>> actually save the 'context' of the deadlocked Dom-U. Please correct 
>> me if this
>> claim is wrong.
>>
>> Also, given that Dom-0 can access the page tables and other 
>> structures of the deadlocked guest,
>> can one of you be able to tell me what changes I need to do to 
>> xm_linux_save( ) (and other related functions) to save the state of 
>> the deadlocked guest without doing any handshake with the guest OS ?
>
> You can get at the consistent state of a guest by pausing it and then 
> reading its state. However, the reason for the handshake is to ensure 
> that the guest is not currently accessing pagetables or doing other 
> critical operations. If it were then we could not safely translate its 
> memory page addresses as it could have those addresses in places like 
> its kernel stacks or register contexts, where they would not get 
> translated and would cause a crash on restore.

I should add that this is a problem specific to writable page tables as 
the guest must be aware of the actual physical pages that it is using.   
With a VT/SVM guest or on an architecture that doesn't use writable page 
tables, this isn't an issue.

Regards,

Anthony Liguoi

>  -- Keir
>