RE: x86/irq: Unbreak interrupt affinity setting

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

From: David Laight
> Sent: 26 August 2020 22:37
> 
> From: Thomas Gleixner
> > Sent: 26 August 2020 21:22
> ...
> > Moving interrupts on x86 happens in several steps. A new vector on a
> > different CPU is allocated and the relevant interrupt source is
> > reprogrammed to that. But that's racy and there might be an interrupt
> > already in flight to the old vector. So the old vector is preserved until
> > the first interrupt arrives on the new vector and the new target CPU. Once
> > that happens the old vector is cleaned up, but this cleanup still depends
> > on the vector number being stored in pt_regs::orig_ax, which is now -1.
> 
> 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.
> 
> 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.
> Worse than that, the hardware could easily only look at the address
> and value in the clocks after checking the interrupt is enabled.
> So masking the interrupt immediately prior to changing the vector
> info may not be enough.
> 
> It is likely that a read-back of the mask before updating the vector
> is enough.

But not enough to assume you won't receive an interrupt after reading
back that interrupts are masked.

(I've implemented the hardware side for an fpga ...)

	David

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