RE: x86/irq: Unbreak interrupt affinity setting

[David Laight <David.Laight@ACULAB.COM>]

From: Thomas Gleixner
> Sent: 26 August 2020 23:08
...
> > I suspect that it is much more 'racy' than that for PCI-X interrupts.
> > On the hardware side there is an interrupt disable bit, and address
> > and a value.
> > To raise an interrupt the hardware must write the value to the
> > address.
> 
> Really?

Yep, anyone with write access to the msi-x table can get the device
to write to any physical location (allowed by any IOMMU) instead of
raising an interrupt.

> > If the cpu needs to move an interrupt both the address and value
> > need changing, but the cpu wont write the address and value using
> > the same TLP, so the hardware could potentially write a value to
> > the wrong address.
> 
> Now I understand finally why msi_set_affinity() in x86 has to be so
> convoluted.

Updating the registers should be much the same on all architectures.
I probably should have looked at what msi_set_affinity() does before
deciding which order the fpga logic should read the four 32bit registers
in; but they are read in increasing order - so enable bit last.

	David

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