Re: [Xen-devel] [PATCHv2 2/3] mm: don't free pages until mm locks are released

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 02/12/15 16:46, Tim Deegan wrote:
> At 16:30 +0000 on 02 Dec (1449073841), George Dunlap wrote:
>> On 02/12/15 16:23, Tim Deegan wrote:
>>> At 07:25 +0000 on 02 Dec (1449041100), Tian, Kevin wrote:
>>>>> From: David Vrabel [mailto:david.vrabel@xxxxxxxxxx]
>>>>> Sent: Saturday, November 14, 2015 2:50 AM
>>>>>
>>>>> If a page is freed without translations being invalidated, and the page is
>>>>> subsequently allocated to another domain, a guest with a cached
>>>>> translation will still be able to access the page.
>>>>>
>>>>> Currently translations are invalidated before releasing the page ref, but
>>>>> while still holding the mm locks.  To allow translations to be invalidated
>>>>> without holding the mm locks, we need to keep a reference to the page
>>>>> for a bit longer in some cases.
>>>>>
>>>>> [ This seems difficult to a) verify as correct; and b) difficult to get
>>>>> correct in the future.  A better suggestion would be useful.  Perhaps
>>>>> using something like pg->tlbflush_needed mechanism that already exists
>>>>> for pages from PV guests? ]
>>>>
>>>> Per-page flag looks clean in general, but not an expert here. Tim might
>>>> have a better idea.
>>>
>>> I think you can probably use the tlbflush_timestamp stuff as-is for
>>> EPT flushes -- the existing TLB shootdowns already drop all EPT
>>> translations.
>>
>> Are you saying that if you do a TLB shootdown you don't need to do an
>> invept command?
> 
> Yes, I think so.  flush_area_local() -> hvm_flush_guest_tlbs() ->
> hvm_asid_flush_core() should DTRT.

I don't think so. Guest-physical mappings are only tagged with the EP4TA
and not the VPID, thus changing the VPID does nothing.

Thus we would need to extend the current mechanism to determine that a
invept is also needed.

David

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