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

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 02/12/15 16:45, David Vrabel 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.
> 
> 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.

That's 28.3.3.2, in case anyone else wants to look for it...

 -George


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