Re: [Xen-devel] Does a Virtual PCI Device can have MSI's

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Wed, 13 Aug 2014, manish jaggi wrote:
> On 13 August 2014 16:20, Stefano Stabellini
> <stefano.stabellini@xxxxxxxxxxxxx> wrote:
> > On Wed, 13 Aug 2014, manish jaggi wrote:
> >> On 13 August 2014 15:40, Stefano Stabellini
> >> <stefano.stabellini@xxxxxxxxxxxxx> wrote:
> >> > On Wed, 13 Aug 2014, manish jaggi wrote:
> >> >> Hi,
> >> >>
> >> >> I think it should be possible, but confirming it that this feature is
> >> >> enabled in xen. I don't know how to test it.
> >> >>
> >> >> Does any virtual PCI device in DomU (I don't mean a virtual function)
> >> >> have MSI interrupts ?
> >> >
> >> > Yes, they do.
> >> >
> >> >
> >> >> If yes then how is that MSI handled in Xen
> >> >
> >> > PV guests on x86 don't receive MSIs or legacy interrupts as is.  They
> >> > map them into "pirqs" instead, that are a kind of event channels, Xen
> >> > specific software interrupts. For each MSI on the PCI device assigned to
> >> > the guest, the guest kernel would ask for a pirq, see:
> >> >
> >> > arch/x86/pci/xen.c:xen_pcifront_enable_irq
> >> > arch/x86/pci/xen.c:xen_setup_msi_irqs
> >> >
> >> > In the specific case of MSIs and MSI-X, pcifront issues an hypercall in
> >> > order to enable them, see:
> >> >
> >> > drivers/pci/xen-pcifront.c:pci_frontend_enable_msi
> >> >
> >> > and the backend returns the pirq number:
> >> >
> >> > drivers/xen/xen-pciback/pciback_ops.c:xen_pcibk_enable_msi
> >> >
> >> >
> >> > On ARM I think it would be best if we delivered MSIs as MSIs to the
> >> > guest, rather than mapping them into pirqs, to take better advantage of
> >> > the hardware. But it would be up to you to change the pcifront/pciback
> >> > code to do it.
> >> > In first instance it would be fine if we end up using pirqs.
> >>
> >> I am considering 2 cases here
> >> a) physical PCI passthrough devices / functions assigned to domU
> >
> > Are you sure you mean DomU here, or maybe you mean Dom0?
> >
> Yes DomU only.
> >
> >> b) emulated (virtual) PCI devices assigned to domU
> >
> > We need to clarify the terminology here: what do you mean by (b)?
> > Emulating an entire PCI device and exposing it to domU? Why do you want
> > to do that? It is not a feature I am keen on having on Xen on ARM.
> > Otherwise if you are thinking of a virtual function of an SR-IOV card,
> > that is still (a) from the Xen point of view.
> 
> I was trying to understand that does Xen support some device like a
> sata device which is a virtual one emulated using qemu on a PV domU
> and its interrupts are MSIs

I wouldn't want to support this use case at all, unless strictly
necessary: emulation is slower and less secure (larger surface of
attack) than PV interfaces.


> >> For (a) it is straight to configure and inject the MSI into guest
> >
> > Yep, that is what I was trying to say.
> >
> >
> >> For (b) how does the configuring and injection should work,
> >> - PCI Front driver using backops requests to enable msi
> >> - At a later stage xen using dom0 (somehow) inject an virtual LPI into 
> >> domU.
> >>
> >> What are your thoughts on this?
> >
> > I am not sure I understand what you mean by (b) anymore. In fact
> > pcifront is used to deal with PCI passthrough to DomUs, that would be
> > (a) by your description.
> 

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