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

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Message-ID: <87msd6y8a7.ffs@tglx>
Date: Thu, 27 Mar 2025 13:20:32 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Neeraj Upadhyay <Neeraj.Upadhyay@....com>, linux-kernel@...r.kernel.org,
 bp@...en8.de, mingo@...hat.com, dave.hansen@...ux.intel.com,
 Thomas.Lendacky@....com, nikunj@....com, Santosh.Shukla@....com,
 Vasant.Hegde@....com, Suravee.Suthikulpanit@....com, David.Kaplan@....com,
 x86@...nel.org, hpa@...or.com, peterz@...radead.org, pbonzini@...hat.com,
 kvm@...r.kernel.org, kirill.shutemov@...ux.intel.com, huibo.wang@....com,
 naveen.rao@....com
Subject: Re: [RFC v2 13/17] x86/apic: Handle EOI writes for SAVIC guests

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.