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Re: [PATCH 1/6] VT-d: properly reserve DID 0 for caching mode IOMMUs
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Fri, 12 Nov 2021 13:19:00 +0100
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, "Kevin Tian" <kevin.tian@xxxxxxxxx>
- Delivery-date: Fri, 12 Nov 2021 12:19:31 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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.
Thanks, Roger.
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