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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Wed, Jun 17, 2015 at 03:26:10PM +0100, Ian Campbell wrote:
> On Wed, 2015-06-17 at 15:18 +0100, Stefano Stabellini wrote:
> > On Wed, 17 Jun 2015, Ian Campbell wrote:
> > > On Wed, 2015-06-17 at 14:40 +0100, Stefano Stabellini wrote:
> > > > On Wed, 17 Jun 2015, Ian Campbell wrote:
> > > > > On Wed, 2015-06-17 at 13:53 +0100, Stefano Stabellini wrote:
> > > > > > On Wed, 17 Jun 2015, Ian Campbell wrote:
> > > > > > > On Tue, 2015-06-16 at 18:16 +0100, Stefano Stabellini wrote:
> > > > > > > > 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.
> > > > > > > 
> > > > > > > Whoever makes the initial decision/setup we still need to decide 
> > > > > > > what to
> > > > > > > do about reads and writes to the device's BAR registers. Who 
> > > > > > > deals with
> > > > > > > these and how, and if writes are allowed and if so how is this 
> > > > > > > then
> > > > > > > reflected in the guest p2m.
> > > > > > > 
> > > > > > > I would very much prefer ARM and x86 PVH to use the same approach 
> > > > > > > to
> > > > > > > this.
> > > > > > > 
> > > > > > > For x86 HVM I believe QEMU takes care of this, by emulating the
> > > > > > > reads/writes to CFG space and making hypercalls (domctls?) to 
> > > > > > > update the
> > > > > > > p2m. There is no QEMU to do this in the ARM or x86 PVH cases.
> > > > > > > 
> > > > > > > For x86 PV I believe pciback deals with the register reads/writes 
> > > > > > > but it
> > > > > > > doesn't need to do anything wrt the p2m because that is a guest 
> > > > > > > internal
> > > > > > > construct. This obviously doesn't apply to ARM or x86 PVH either 
> > > > > > > since
> > > > > > > the p2m is managed by Xen in both those cases.
> > > > > > > 
> > > > > > > So it seems that neither of the existing solutions are a good 
> > > > > > > match for
> > > > > > > either ARM or x86 PVH.
> > > > > > > 
> > > > > > > _If_ it were permissible to implement all BAR registers as r/o 
> > > > > > > then this
> > > > > > > might simplify things, however I suspect this is not something we 
> > > > > > > can
> > > > > > > get away with in the general case (i.e. a driver for a given PCI 
> > > > > > > device
> > > > > > > _knows_ that BAR<N> is writeable...).
> > > > > > > 
> > > > > > > So I think we do need to allow guest writes to the BAR registers. 
> > > > > > > I
> > > > > > > think it would be a bit strange (or even open potentially subtle
> > > > > > > (security?) issues) to require the guest to write the BAR 
> > > > > > > register and
> > > > > > > to update the p2m to match as well.
> > > > > > 
> > > > > > Why would it be a security risk?
> > > > > 
> > > > > No idea, it seemed like a vague possibility. I did say "(security?)" 
> > > > > not
> > > > > "SECURITY" ;-)
> > > > > 
> > > > > >  I don't think it is important that the
> > > > > > interface between pcifront and pciback refrects the hardware 
> > > > > > interface.
> > > > > > But it is important that the interface is documented and complies 
> > > > > > to the
> > > > > > expected behaviour.
> > > > > > 
> > > > > > I think that if we go with XENMEM_add_to_physmap_range, it would be 
> > > > > > OK
> > > > > > for pcifront to both write to the vBAR (actually writing to the 
> > > > > > ring to
> > > > > > pciback), then call XENMEM_add_to_physmap_range.
> > > > > > 
> > > > > > On the other hand, if we go with XEN_DOMCTL_memory_mapping, I would 
> > > > > > make
> > > > > > the virtual BAR read-only, that is less preferable but still 
> > > > > > acceptable,
> > > > > > as long as we document it.
> > > > > 
> > > > > Given that this appears to be how pv pci works today I think this 
> > > > > turns
> > > > > out to be preferable and saves us having to argue the toss about who
> > > > > should do the p2m updates on a BAR update.
> > > > > 
> > > > > Which would point towards the toolstack doing the layout and telling
> > > > > pciback somehow what the bars for each device are and that's it.
> > > > > 
> > > > > (making most of the rest of your reply moot, if you think its really
> > > > > important to let guests move BARs about I can go through it again)
> > > > 
> > > > I think is unimportant, in fact having the guest setup the mappings was
> > > > my original suggestion.
> > > 
> > > I can't reconcile the two halves of this statement, are you saying you
> > > are still in favour of the guest dealing with the mappings or not?
> > 
> > Sorry!
> > 
> > I do not think is important to let guests move BARs. In fact having the
> > toolstack doing the layout was my original suggestion and I still think
> > can work correctly.
> 
> Ah, so s/guest/toolstack/ on the statement above makes it consistent,
> phew!
> 
> > I don't have a strong feeling on whether it should be the toolstack or
> > the guest to do the mapping. It can work either way. I would certainly
> > avoid having Dom0 do the mapping.
> 
> I think it should be the toolstack. Allow the guest to do it means
> implementing writeable BARs which is a much bigger yakk to shave and it
> doesn't seem to be necessary.

True. I skipped having to implement this for PV guests by having the
PV + PCI passthrough use the host E820. Which means that the PV guest would
redo its P2M tree to match the E820. That nicely setup the P2M to have 1-1
mappings for the MMIO regions.

This would imply that on PVH + PCI we need to setup the E820 to be similar
to the host. I don't know what it means on ARM memory layout?

It also means that migration is a bit crippled - as even if you unplug
the PCI device, and then plug it back on the other end - the E820 might
be very much different.


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