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

[manish jaggi <manishjaggi.oss@xxxxxxxxx>]

On 13 August 2014 19:13, Stefano Stabellini
<stefano.stabellini@xxxxxxxxxxxxx> wrote:
> 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.
>
Thats why I asked this question, if a virtual device is assigned to a
domU how would the MSIs be configured for it, using front-back
communication or using the Linux ITS driver which gets trapped into
Xens ITS driver.
>
>> >> 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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