Re: x86/irq: Unbreak interrupt affinity setting

[Alexander Graf <graf@amazon.com>]

On 26.08.20 23:47, David Laight wrote:
> 
> 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 ...)

Do we actually care in this context? All we want to know here is whether 
a device (or irqchip in between) has actually noticed that it should 
post to a new vector. If we get interrupts on random other vectors in 
between, they would simply show up as spurious, no?

So I don't quite see where this patch makes the situation any worse than 
before.


Alex



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