Re: [Xen-devel] [RFC PATCH 11/24] ARM: vGICv3: handle virtual LPI pending and property tables

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 2 Nov 2016, Julien Grall wrote:
> Hi Stefano,
> 
> On 02/11/16 17:41, Stefano Stabellini wrote:
> > On Wed, 2 Nov 2016, Julien Grall wrote:
> > > > +    }
> > > > +
> > > > +    ptr = vmap(pages, nr_pages);
> > > 
> > > I am not a big fan of the vmap solution for various reasons:
> > >   - the VMAP area is small (only 1GB) it will not scale (you seem
> > > to use it to map pretty much all memory provisioned for the ITS)
> > >   - writing in a register cannot fail, how do you co-op with that?
> > > 
> > > I think the best approach here is to use a similar approach as
> > > copy_*_guests helpers but dealing with IPA rather than guest VA.
> > 
> > I don't like the idea of using the vmap for this either, but the problem
> > with the copy_*_guest approach is that it only maps one page at a time.
> > It is unable to map multiple pages contiguously, which seems to be
> > required here.
> 
> We will get into trouble very quickly with the vmap solution. The memory
> provisioned by the ITS can be quite big. Each GITS_BASER can hold up to 16MB
> (if I computed correctly) of memory.
> 
> The question is why do we need to map the page contiguously? Can we do in a
> different way?
 
I agree with Julien: if we can get away with mapping one page at a time
without performance penalties, we should do it. Keep in mind that on
ARM64 Xen doesn't actually need to map anything because the whole
physical memory is already mapped.

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