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

[Tim Deegan <tim@xxxxxxx>]

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.

Tim.


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