Re: [Xen-devel] PCI Passthrough ARM Design : Draft1

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Tue, 16 Jun 2015, Roger Pau Monnà wrote:
> El 16/06/15 a les 18.13, Stefano Stabellini ha escrit:
> > On Thu, 11 Jun 2015, Ian Campbell wrote:
> >> On Thu, 2015-06-11 at 07:25 -0400, Julien Grall wrote:
> >>> Hi Ian,
> >>>
> >>> On 11/06/2015 04:56, Ian Campbell wrote:
> >>>> On Wed, 2015-06-10 at 15:21 -0400, Julien Grall wrote:
> >>>>> Hi,
> >>>>>
> >>>>> On 10/06/2015 08:45, Ian Campbell wrote:
> >>>>>>> 4. DomU access / assignment PCI device
> >>>>>>> --------------------------------------
> >>>>>>> When a device is attached to a domU, provision has to be made such 
> >>>>>>> that
> >>>>>>> it can
> >>>>>>> access the MMIO space of the device and xen is able to identify the 
> >>>>>>> mapping
> >>>>>>> between guest bdf and system bdf. Two hypercalls are introduced
> >>>>>>
> >>>>>> I don't think we want/need new hypercalls here, the same existing
> >>>>>> hypercalls which are used on x86 should be suitable. That's
> >>>>>> XEN_DOMCTL_memory_mapping from the toolstack I think.
> >>>>>
> >>>>> XEN_DOMCTL_memory_mapping is done by QEMU for x86 HVM when the guest
> >>>>> (i.e hvmloader?) is writing in the PCI BAR.
> >>>>
> >>>> What about for x86 PV? I think it is done by the toolstack there, I
> >>>> don't know what pciback does with accesses to BAR registers.
> >>>
> >>> XEN_DOMCTL_memory_mapping is only used to map memory in stage-2 page
> >>> table. This is only used for auto-translated guest.
> >>>
> >>> In the case of x86 PV, the page-table is managed by the guest. The only
> >>> things to do is to give the MMIO permission to the guest in order to the
> >>> let him use them. This is done at boot time in the toolstack.
> >>
> >> Ah yes, makes sense.
> >>
> >> Manish, this sort of thing and the constraints etc should be discussed
> >> in the doc please.
> >
> > I think that the toolstack (libxl) will need to call
> > xc_domain_memory_mapping (XEN_DOMCTL_memory_mapping), in addition to
> > xc_domain_iomem_permission, for auto-translated PV guests on x86 (PVH)
> > and ARM guests.
>
> I'm not sure about this, AFAICT you are suggesting that the toolstack
> (or domain builder for Dom0) should setup the MMIO regions on behalf of
> the guest using the XEN_DOMCTL_memory_mapping hypercall.
>
> IMHO the toolstack should not setup MMIO regions and instead the guest
> should be in charge of setting them in the p2m by using a hypercall (or
> at least that was the plan on x86 PVH).

I wrote this before reading the rest of the thread with your alternative
suggestion.  Both approaches can work, maybe it is true that having the
guest requesting mappings is better. And we should certainly do the same
thing for PVH and ARM guests.

If we have the guest requesting the mapping, obviously the hypercall
implementation should check whether mapping the memory region has been
previously allowed by the toolstack, that should still call
xc_domain_iomem_permission.

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