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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

Could you please use plain text emails?  See how bad it looks in my
client.

One comment below.

On Tue, 16 Jun 2015, Manish Jaggi wrote:
> On Tuesday 16 June 2015 10:28 AM, Stefano Stabellini wrote:
>
> On Tue, 16 Jun 2015, Manish Jaggi wrote:
>
> On Tuesday 16 June 2015 09:21 AM, 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).
>
> Roger.
>
> There were couple of points discussed,
> a) There needs to be a hypercall issued from an entity to map the device MMIO
> space to domU.
> What that entity be
>  i) Toolstack
> ii) domU kernel.
>
> b) Should the MMIO mapping be 1:1
>
> For (a) I have implemented in domU kernel in the context of the notification
> received when a device is added on the pci-front bus. This was a logical point
> I thought this hypercall should be called. Keep in mind that I am still not
> aware how this works on x86.
>
> I think that is OK, but we would like to avoid a new hypercall. Roger's
> suggestion looks good.
>
> Roger,
> Âas per your comment if guest is charge of setting p2m, which existing 
> hypercall to be used ?
>
> For (b) The BAR region is not updated AFAIK by the pci device driver running
> in domU. So once set the BARs by firmware or enumeration logic, are not
> changed, not in domU for sure. Then it is 1:1 always.
> Should the BAR region of the device be updated to make it not 1:1 ?
>
> I think the point Ian and Julien were trying to make is that we should
> not rely on the mapping being 1:1. It is OK for the guest not to change
> the BARs. But given that the memory layout of the guest is different
> from the one on the host, it is possible that the BAR might have an
> address that overlaps with a valid memory range in DomU. In that case
> the guest should map the MMIO region elsewhere in the guest physical
> space. It might also want to update the virtual BAR accordingly.
>
> (Alternatively the toolstack could come up with an appropriate placement
> of the virtual BARs and MMIO region mappings in the guest.
>
> should the pci-back driver return a virtual BAR in response to a pci conf 
> space read from the domU.

Yes, pciback is already capable of doing that, see
drivers/xen/xen-pciback/conf_space.c

> I am not sure if the pci-back driver can query the guest memory map. Is there 
> an existing hypercall ?

No, that is missing.  I think it would be OK for the virtual BAR to be
initialized to the same value as the physical BAR.  But I would let the
guest change the virtual BAR address and map the MMIO region wherever it
wants in the guest physical address space with
XENMEM_add_to_physmap_range.

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