Re: [RFC v2 13/17] x86/apic: Handle EOI writes for SAVIC guests

Thomas Gleixner
2025-03-27T12:20:34Z

On Thu, Mar 27 2025 at 11:48, Thomas Gleixner wrote:

> On Fri, Mar 21 2025 at 10:11, Sean Christopherson wrote:
>> On Fri, Mar 21, 2025, Thomas Gleixner wrote:
>>>
>>> Congrats. You managed to re-implement find_last_bit() in the most
>>> incomprehesible way.
>>
>> Heh, having burned myself quite badly by trying to use find_last_bit() to get
>> pending/in-service IRQs in KVM code...
>>
>> Using find_last_bit() doesn't work because the ISR chunks aren't contiguous,
>> they're 4-byte registers at 16-byte strides.
>
> Which is obvious to solve with trivial integer math:
>
> bit = vector + 32 * (vector / 32);
>
> ergo
>
> vector = bit - 16 * (bit / 32);
>
> No?

Actually no. As this is for 8 byte alignment. For 16 byte it's

bit = vector + 96 * (vector / 32);
ergo
vector = bit - 24 * (bit / 32);

Which is still just shifts and add/sub.