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

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Thu, Dec 3, 2015 at 10:22 AM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>>> On 02.12.15 at 18:14, <david.vrabel@xxxxxxxxxx> wrote:
>> 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.
>
> Indeed. And in fact they tell us that using VPID together with EPT is
> kind of pointless when different guests use different EPTPs.

Except that of course, that they allow us to isolate the flushing of
combined mappings from between guests...

 -George

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