Re: [Xen-devel] [PATCH] amd iommu: Do not adjust paging mode for dom0 devices

[Wei Wang2 <wei.wang2@xxxxxxx>]

On Monday 07 February 2011 11:47:32 Keir Fraser wrote:
> On 07/02/2011 10:33, "Wei Wang2" <wei.wang2@xxxxxxx> wrote:
> >> And that's wrong is it? How do you know that dom0 doesn't have a whole
> >> load of memory assigned to it?
> >>
> >> The correct thing to do would be to adjust the table depth according to
> >> the largest page number currently mapped in the table. Or just stick
> >> with four levels always if you can't do the optimisation job properly.
> >>
> >>  -- Keir
> >
> > Keir,
> > I was a little confused, are you suggesting that max_page does not
> > represent the last pfn of dom0?
>
> The global variable max_page represents the largest machine frame number in
> the system.
Yes, that is also my assumption

> The domain field d->max_pages merely represents an allocation limit for a
> domain, beyond which further allocation requests will be refused. Note it
> doesn't guarantee that the domain does not have less memory, or *more*
> memory (if max_pages got reduced below a domain's current allocation).
>
> Also, for a PV guest like dom0, where the IOMMU table is presumably a 1:1
> mapping, d->max_pages is not useful in any case because even if a guest
> has, say, only 100MB memory allocated to it, that memory can be spread
> across the entire host memory space from 0 to max_page. And max_page could
> be big!
OK, I misunderstood it. I thought d->max_pages also stands for last gfn for 
domU like max_page for the whole system.

> Personally I would suggest starting with small 2-level tables and
> dynamically increase their height as bigger mappings are added to them.
> Else stick with 4-level tables, or size tables according to global variable
> max_page. I think basing anything on d->max_pages is not a good idea.
>
>  -- Keir
How does the attached patch look like? It uses global variable max_page for pv 
and dom0 and calculate maxpfn for hvm guest. This should cover gfn holes on 
hvm guests.

Thanks,
Wei
Signed-off-by: Wei Wang <wei.wang2@xxxxxxx>

> > I was assuming max_pdx is the index number... Or are
> > you referring memory hot plug? If so, we might also need 4 level for
> > dom0.

Attachment: fix_pg_mode.patch
Description: fix_pg_mode.patch

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