Re: [PATCH for-4.21 03/10] x86/HPET: use single, global, low-priority vector for broadcast IRQ

[Jan Beulich <jbeulich@xxxxxxxx>]

On 16.10.2025 19:01, Andrew Cooper wrote:
> On 16/10/2025 8:32 am, Jan Beulich wrote:
>> Using dynamically allocated / maintained vectors has several downsides:
>> - possible nesting of IRQs due to the effects of IRQ migration,
>> - reduction of vectors available for devices,
>> - IRQs not moving as intended if there's shortage of vectors,
>> - higher runtime overhead.
>>
>> As the vector also doesn't need to be of any priority (first and foremost
>> it really shouldn't be of higher or same priority as the timer IRQ, as
>> that raises TIMER_SOFTIRQ anyway), avoid any "ordinary" vectors altogther
>> and use a vector from the 0x10...0x1f exception vector space. Exception vs
>> interrupt can easily be distinguished by checking for the presence of an
>> error code.
>>
>> Fixes: 996576b965cc ("xen: allow up to 16383 cpus")
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> This is so cunning that it took me a while to figure out how this even
> functioned, given no apparent change to TPR.

Yeah, the TPR part took me a while to figure out. I was scratching my head
as to why I wasn't seeing any interrupts, until I figured I need to make
that adjustment to FIRST_IRQ_VECTOR.

> Having this behaviour under FRED is easy.  In fact, allowing the use of
> vectors 0x10-0x1f under FRED is one "extra" I haven't gotten around to
> doing yet.

Ah yes.

> But, the problem it introduces under IDT is that for all the other
> reserved exceptions, we'll panic if we see them.  That was the point of
> setting TPR to 0x10 originally.

Which would be a clear indication of something being wrong, rather than
things misbehaving (likely) entirely silently. So more of a benefit than
a downside?

Of course, if we were afraid of other vectors getting signaled, other
entry points may need amending similarly to what I do to entry_AC(). It
then isn't quite clear to me what, if anything, to do to entry points of
exceptions coming without error code.

Jan