Re: [Xen-devel] [v5][PATCH 03/10] xen:x86: define a new hypercall to get RMRR mappings

["Chen, Tiejun" <tiejun.chen@xxxxxxxxx>]

On 2014/9/2 21:15, Jan Beulich wrote:
> > > > On 02.09.14 at 13:10, <tiejun.chen@xxxxxxxxx> wrote:
> > On 2014/9/2 18:15, Jan Beulich wrote:
> > > > > > On 02.09.14 at 11:59, <tiejun.chen@xxxxxxxxx> wrote:
> > > > +    case XENMEM_reserved_device_memory_map:
> > > > +    {
> > > > +        struct xen_mem_reserved_device_memory *xmrdm = NULL;
> > > > +        struct xen_mem_reserved_device_memory_map xmrdmm;
> > > > +        XEN_GUEST_HANDLE(xen_mem_reserved_device_memory_t) buffer;
> > > > +        XEN_GUEST_HANDLE_PARAM(xen_mem_reserved_device_memory_t) buffer_param;
> > > > +        const struct iommu_ops *ops = iommu_get_ops();
> > > > +        unsigned int nr_entries = 0;
> > > > +        unsigned int i = 0;
> > > > +
> > > > +        xmrdm = ops->get_device_reserved_memory(&nr_entries);
> > Do we still need this iommu_ops somewhere else?
> Not this one, but another one (as I had described further down).
>
> > > > +        if ( !nr_entries )
> > Do we still need this 'nr_entries' here?
> Doesn't look like so. But it's you coding this up - you ought to clearly
> see what is needed and what not.
I mean we need to get 'nr_entries' before any real operations since if 
that is zero, any following operations are pointless. And even that is 
nonzero, but actually the user don't know how many buffer should be 
passed, so often we may need to notify user again at the first time then 
the user call this again with a appropriate buffer. Right?
So how do we get this 'nr_entries'? Seems we have to to go VT-D stuff to 
walk that list or other similar approaches. If so, why we still get 
those real mapping entries later by accessing VT-D stuff again?
Shouldn't we figure out a approach we just call one time?

Thanks
Tiejun

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