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 16:23, 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.

Regular TLB invalidate instructions (e.g., INVLPG) only invalidate
linear and combined mappings.  guest-physical mappings are not affected
and require an INVEPT.

Section 2.3.3.2 of the Intel SDM vol 3 gives a useful summary.

David

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