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

From: Alexander Gordeev
> > 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.
> Anyway, as I described in my reply to Bjorn, this is not a concern IMO.

I'm thinking of the following - which might be MSI-X ?
1) Hardware requests some interrupts and tells the host the BAR (and offset)
   where the 'vectors' should be written.
2) To raise an interrupt the hardware uses the 'vector' as the address
   of a normal PCIe write cycle.

So if the hardware requests 4 interrupts, but the driver (believing it
will only use 3) only write 3 vectors, and then the hardware uses the
4th vector it can write to a random location.

Debugging that would be hard!


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