Re: [Xen-devel] [rfc 00/18] ioemu: use devfn instead of slots as the unit for passthrough

[Yuji Shimada <shimada-yxb@xxxxxxxxxxxxxxx>]

On Mon, 23 Feb 2009 17:55:30 +1100
Simon Horman <horms@xxxxxxxxxxxx> wrote:

> On Mon, Feb 23, 2009 at 03:24:41PM +0900, Yuji Shimada wrote:
> > On Fri, 20 Feb 2009 18:07:00 +1100
> > Simon Horman <horms@xxxxxxxxxxxx> wrote:
> > 
> > > On Thu, Feb 19, 2009 at 09:38:24AM +0000, Keir Fraser wrote:
> > > > On 19/02/2009 09:21, "Yuji Shimada" <shimada-yxb@xxxxxxxxxxxxxxx> wrote:
> > > > 
> > > > >> To be honest I am a little confused about what the above maping
> > > > >> is supposed to achive.
> > > > > 
> > > > > Please find the attached figure which shows the interrupt routing in
> > > > > xen hypervisor.
> > > > 
> > > > The point being to deliberately permute the mapping to try to avoid
> > > > accidental GSI sharing even if there are patterns in DEV:INTX usage 
> > > > (e.g.,
> > > > all devs use INTA).
> > > 
> > > Thanks for the information, especially the diagram. It is very useful.
> > > 
> > > Armed with this new kowledge I have a few questions.
> > > 
> > > 1. Shimada-san stated that shared GSI are not permitted for
> > >    pass-through devices. Is it permitted for a GSI to be shared
> > >    between a pass-through device and a non-pass-through device?
> > 
> > Yes, it is permitted. But guest software will receive spurious
> > interrupt. So it is not good.
> 
> Ok, so it would be good to avoid if possible.
> 
> > >    The current scheme seems to leave scope for this as
> > > 
> > >    gsi 6 A = gsi 13 D = gsi 21 C = gsi 29 B
> > >    gsi 7 A = gsi 14 D = gsi 22 C = gsi 30 B
> > 
> > Do you mean this?
> > 
> >      Dev 6 INTA = Dev 13 INTD = Dev 21 INTC = Dev 29 INTB -> GSI 40
> >      Dev 7 INTA = Dev 14 INTD = Dev 22 INTC = Dev 30 INTB -> GSI 44
> 
> Yes, that is what I meant.
> 
> > > 2. In several places in ioemu:io/passthrough.c e_intx is set to 0,
> > >    corresponding to INTA. Is this because it is virtual and
> > >    using INTA is convenient? Or is it because it is assumed
> > >    that the physical device being passed-through is a 0 function
> > >    (and 0 functions always use INTA) ?
> > 
> > INTx is virtualized, because the single function device normally use
> > INTA.
> 
> Suppose the case where 00:1d.0 has INTA and 00:1d.1 has INTB,
> and both these functions are passed-trhough into a guest
> without any of my patches applied. In the guest 00:1d.0 will
> appear as 00:06.0 with INTA. And 00:1d.1 will apepar as
> 00:06.1 with INTA. Is this ok?

00:1d.1 with INTB will appear as 00:07.0 with INTA, when we use
current xen.

> > When we make multi-function cards appear in guests as multi-function
> > cards, it is good that virtual INTx reflects the physical INTx. The
> > reason is one of functions of a device may share INTx of the other
> > function. In my environment, UHCI(00:1d.0) and EHCI(00:1d.7) share the
> > same INTA. If physical functions share physical INTx, virtual
> > functions should share virtual INTx. To achieve this, virtual INTx
> > needs to reflect the physical INTx.
> 
> Understood. The issue of assigning GSIs aside, this should
> be fairly straightforward.
> 
> > >    The latter assumption is not valid because even without my pacthes
> > >    it is possible to pass-through non-0 functions, its just that
> > >    they end up as the 0th function of the virtual slot in the guest.
> > > 
> > > I am now pretty sure that my change leads to incorrect usage of
> > > hvm_pci_intx_gsi(). Answers to the questions above will help me to
> > > understand how trivial to fix this is.
> > > 
> > > The most difficult cases seem to be 1) sharing of gsi between
> > > pass-through and non-pass-through devices is not permitted or 2)
> > > intx used inside ioemu:io/passthrough.c should reflect the physical
> > > intx. In either case I wonder if a reasonable solution would be to
> > > just allocate allocate GSI in a non-colliding manner. Say, GSI 16 for
> > > the first device to ask, 17 for the next one and so on. Or perhaps
> > > the existing hash + overflow to the next GSI on collision.
> > 
> > The another solution is expanding GSI to 127. I don't sure it is
> > possible, but sharing virtual GSI will not occur.
> 
> That thought crossed my mind too, I will investigate further.
> But I think that ideally it would need to be expanded to 143
> as the first 16 GSI are currently reserved for ISA.

That's exactly right.

Thanks,
--
Yuji Shimada

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