Re: [Xen-devel] [PATCH] x86/p2m: Stop other vcpus using a nested p2m before clearing it

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 08/02/17 13:20, Andrew Cooper wrote:
> On 08/02/17 13:13, Jan Beulich wrote:
>>>>> On 07.02.17 at 19:48, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> Until the IPI has completed, other processors might be running on this 
>>> nested
>>> p2m object.  clear_domain_page() does not guarantee to make 8-byte atomic
>>> updates, which means that a pagewalk on a remote processor might encounter a
>>> partial update.
>>>
>>> This is currently safe as other issues prevents a nested p2m ever being 
>>> shared
>>> between two cpus (although this is contrary to the original plan).
>>>
>>> Setting p2m->np2m_base to P2M_BASE_EADDR before the IPI ensures that the 
>>> IPI'd
>>> processors won't continue to use the flushed mappings.
>>>
>>> While modifying this function, remove all the trailing whitespace and tweak
>>> style in the affected areas.
>>>
>>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
>> but ...
>>
>>> @@ -1633,19 +1635,21 @@ p2m_flush_table(struct p2m_domain *p2m)
>>>  
>>>      /* This is no longer a valid nested p2m for any address space */
>>>      p2m->np2m_base = P2M_BASE_EADDR;
>>> -    
>>> -    /* Zap the top level of the trie */
>>> -    mfn = pagetable_get_mfn(p2m_get_pagetable(p2m));
>>> -    clear_domain_page(mfn);
>>>  
>>>      /* Make sure nobody else is using this p2m table */
>>>      nestedhvm_vmcx_flushtlb(p2m);
>>>  
>>> +    /* Zap the top level of the trie */
>> s/trie/tree/ here, as you touch it anyway?
> 
> Trie here refers to the datastructure https://en.wikipedia.org/wiki/Trie
> which is the structure implemented by processor pagetables.  It is more
> specific than just calling them trees.

Never heard that before, but we seem to already have at least six
instances in the hypervisor (and an ocaml file called 'trie.mli'), so I
guess there's precedent. :-)

Reviewed-by: George Dunlap <george.dunlap@xxxxxxxxxx>

I'll check this one in.

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