Re: [Xen-devel] [PATCH 0/5] Add MSI support to XEN

[Espen Skoglund <espen.skoglund@xxxxxxxxxxxxx>]

That is true.  I was quite puzzled with the requirement of the
callback into Xen myself.  In standard Linux MSI interrupts are
treated as edge triggered and are just acked in the local APIC upon
delivery.

        eSk



[Keir Fraser]
> This requires the guest to call back into Xen to signal EOI (as we already
> do for legacy level-triggered interrupts). We shouldn't really need to do
> that for MSI and it's rather more expensive than a couple of accesses over
> the PCI bus!

> It's this callback into Xen, which we do not really understand why it's
> needed, which I'm railing against. Is there some fundamental aspect of MSI
> we do not understand, or are we working around one brain-dead or buggy
> device?

>  -- Keir

> On 28/3/08 01:48, "Jiang, Yunhong" <yunhong.jiang@xxxxxxxxx> wrote:

>> Not masking each time when interrupt happen, instead, we do that only
>> when the second interrupt happen while the previous one is still
>> pending, it should be something like handle_edge_irqs() in upstream
>> linux.
>> 
>> -- Yunhong Jiang
>> 
>> Espen Skoglund <mailto:espen.skoglund@xxxxxxxxxxxxx> wrote:
>>> Preventing interrupt storms by masking the interrupt in the MSI/MSI-X
>>> capabilty structure or MSI-X table within the interrupt handler is
>>> insane.  It requires accesses over the PCI/PCIe bus and is clearly
>>> something you want to avoid on the fast path.
>>> 
>>> eSk
>>> 
>>> 
>>> [Haitao Shan]
>>>> There are no much changes made compared with the original
>> patches.
>>>> But there do have some issues that we need your kind comments.
>>> 
1> ACK-NEW method is necessary to avoid IRQ storm. But it causes
>> the
>>>> deadlock. During my tests, I do find there can be deadlock
>> with
>>>> patches applied. When assigned a NIC device to HVM domain, the
>> scenario
>>>> is: Dom0 is waiting to IDE interrupt (vector 0x21); HVM domain is
>> waiting
>>>> for qemu's IDE emulation and thus blocked; NIC interrupt (MSI vector
>> 0x31)
>>>> is waiting for injection to HVM domain since it is blocked now; IDE
>>>> interrupt is waiting for NIC interrupt since NIC interrupt is of high
>>>> priority but not ACKed by XEN now. When IDE interrupt and NIC
>> interrupt
>>>> are delivered to the same CPU, and when guest OS is Vista, the
>>>> phenomenon is easy to be observed.
>>> 
2> Without ACK-NEW, some naughty NIC devices as we observed will
>>>> bring IRQ storms. For this phenomenon, I think Yunhong can comment
>> more.
>>>> Basically, writing EOI without mask the source of MSI will bring IRQ
>>>> storm. Although the reason is under investigation, XEN should anyhow
>>>> handle such bogous device, right?
>>> 
3> Using ACK-OLD and masking the MSI when writing EOI can be
>>>> solution. However, XEN does not own PCI configuration spaces.
>> 
>> _______________________________________________
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>> http://lists.xensource.com/xen-devel




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