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RE: [PATCH 1/6] VT-d: properly reserve DID 0 for caching mode IOMMUs
- To: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>
- From: "Tian, Kevin" <kevin.tian@xxxxxxxxx>
- Date: Mon, 15 Nov 2021 05:13:34 +0000
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Mon, 15 Nov 2021 05:13:54 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Thread-index: AQHX16ptxP2kemn3HkOptj6NHfmn+qv/wDwAgAAMLoCAAAMzAIAEP/6g
- Thread-topic: [PATCH 1/6] VT-d: properly reserve DID 0 for caching mode IOMMUs
> From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> Sent: Friday, November 12, 2021 8:19 PM
>
> On Fri, Nov 12, 2021 at 01:07:33PM +0100, Jan Beulich wrote:
> > On 12.11.2021 12:23, Roger Pau Monné wrote:
> > > On Fri, Nov 12, 2021 at 10:47:59AM +0100, Jan Beulich wrote:
> > >> Merely setting bit 0 in the bitmap is insufficient, as then Dom0 will
> > >> still have DID 0 allocated to it, because of the zero-filling of
> > >> domid_map[]. Set slot 0 to DOMID_INVALID to keep DID 0 from getting
> > >> used.
> > >
> > > Shouldn't the whole domid_map be initialized to DOMID_INVALID to
> > > prevent dom0 matching against any unused slot?
> > >
> > > Similarly cleanup_domid_map should set the slot to DOMID_INVALID.
> >
> > I don't think so, that's the purpose of setting the bit in domid_bitmap.
> > The problem really was only with setting a bit in that bitmap without
> > invalidating the corresponding slot.
> >
> > This said, I can still see value in doing as you suggest, but as a
> > separate change with a different justification. In fact domid_bitmap is
> > kind of redundant now anyway; aiui it was the thing that existed first.
> > Then domid_map[] was simply added, rather than fully replacing the
> > original bitmap.
>
> I guess using domid_bitmap to find a free slot is faster than scanning
> the array of iommu IDs to domids. Not sure how performance critical
> that search is, so maybe it's fine to just drop domid_bitmap and rely
> exclusively on the array.
>
I'm fine to drop domid_bitmap. I don't think domain creation
is in hot path...
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