Re: [Xen-devel] [v6][PATCH 2/2] xen:vtd: missing RMRR mapping while share EPT

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Tim Deegan [mailto:tim@xxxxxxx]
> Sent: Thursday, October 02, 2014 3:30 AM
> 
> At 08:07 +0100 on 30 Sep (1412060848), Jan Beulich wrote:
> > >>> On 30.09.14 at 05:49, <yang.z.zhang@xxxxxxxxx> wrote:
> > > Jan Beulich wrote on 2014-09-26:
> > >>>>> On 26.09.14 at 03:24, <yang.z.zhang@xxxxxxxxx> wrote:
> > >>> Jan Beulich wrote on 2014-09-25:
> > >>>>>>> On 25.09.14 at 04:30, <yang.z.zhang@xxxxxxxxx> wrote:
> > >>>>> How do the checking in P2M updates? Only hvmloader knows whether
> > >>>>> the RMRR region is reserved. If we want do the check in
> > >>>>> hypervisor, we need to report those info to hypervisor.
> > >>>>
> > >>>> First of all the hypervisor has the information - it is where the
> > >>>> information comes from that tools and hvmloader consume. And then
> > >>>> the check will need to be a collision check: If while establishing
> > >>>> an identity mapping another mapping is found to be already in
> > >>>> place, the request will need to be failed. And similarly if a
> > >>>> "normal" mapping request finds a 1:1 mapping already in place, this
> > >>>> ought to result in failure too. Of course a prerequisite to this is
> > >>>> that error get properly
> > >>> bubbled up through all layers.
> > >>>
> > >>> If there is another mapping already there and it is different from
> > >>> the one we are establishing, we should return failure. But if the
> > >>> existing mapping is same to the one we are establishing, how we can
> > >>> know whether it is a 'normal memory 1:1' mapping or it is created by
> > >>> previous operation of RMRR creating(e.g. there already a device with
> > >>> RMRR assigned to this guest). What I am thinking is that we need a
> > >>> flag to know whether the mapping is RMRR mapping or regular memory
> > >> mapping.
> > >>
> > >> If the new and old mappings are the same, nothing needs to be done at
> > >> all (as is already the case in one direction in the patches we have
> > >> seen). And yes, for the case when there is an occasional 1:1 mapping
> > >> we of course will need some way of identifying intentional ones.
> > >
> > > So how about adding a new p2m type to do this? It may also helpful when
> > > creating a guest without device attached.(I mentioned it in another
> thread).
> >
> > If that makes it easier to implement - why not? But I think you're aware
> > that raising memory management related questions without Tim in the
> > loop isn't going to yield a result you can reasonably rely on later being
> > accepted in patch form.
> 
> Although adding new p2m types is generally OK, I'm not sure I see the
> need here.  The useful cases are trivial:
>  - gfn space unoccupied -> insert mapping; success.
>  - gfn space already occupied by 1:1 RMRR mapping -> do nothing; success.
>  - gfn space already occupied by other mapping -> fail.
> 
> The remaining case is where the gfn space happens to be entirely
> occupied by an existing 1:1 mapping of RMRR that wasn't put there by
> RMRR code.  If you really want to detect this I think a simple
> reference count of how may times this VM has had the RMRR mapped will
> do.  If that count is 0 and you see a mapping, fail (even if the
> mapping looks right).
> 

I can't consider a valid case for above scenario, i.e. an existing 1:1
mapping of RMRR that wasn't put there by RMRR code. If there is,
it has to be a bug. The only valid case we'll see is that two devices
have same RMRR range, and two devices are assigned to the same
VM. In such case, assignment of the 2nd device would see an existing
RMRR identity mapping, but then it's desired.

So I'd agree with the simple policy of what Tim suggested:
>  - gfn space unoccupied -> insert mapping; success.
>  - gfn space already occupied by 1:1 RMRR mapping -> do nothing; success.
>  - gfn space already occupied by other mapping -> fail.

And then the reference code should be in the per-VM RMRR structure.

Thanks
Kevin

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