Re: [Xen-devel] [PATCH 1/3] PCI/MSI: Add pci_enable_msi_partial()

[David Laight <David.Laight@xxxxxxxxxx>]

From: Bjorn Helgaas
...
> >> Even if you do that, you ought to write valid interrupt information
> >> into the 4th slot (maybe replicating one of the earlier interrupts).
> >> Then, if the device does raise the 'unexpected' interrupt you don't
> >> get a write to a random kernel location.
> >
> > I might be missing something, but we are talking of MSI address space
> > here, aren't we? I am not getting how we could end up with a 'write'
> > to a random kernel location when a unclaimed MSI vector sent. We could
> > only expect a spurious interrupt at worst, which is handled and reported.
> 
> Yes, that's how I understand it.  With MSI, the OS specifies the a
> single Message Address, e.g., a LAPIC address, and a single Message
> Data value, e.g., a vector number that will be written to the LAPIC.
> The device is permitted to modify some low-order bits of the Message
> Data to send one of several vector numbers (the MME value tells the
> device how many bits it can modify).
> 
> Bottom line, I think a spurious interrupt is the failure we'd expect
> if a device used more vectors than the OS expects it to.

So you need to tell the device where to write in order to raise the
'spurious interrupt'.

        David

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