linux-kernel - Re: [PATCH] kvm: x86: simplify kvm_vector_to

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Message-ID: <2927ccc7-07f2-47c9-a902-e66114ea8020@suse.com>
Date: Wed, 20 Aug 2025 16:01:22 +0200
From: Juergen Gross <jgross@...e.com>
To: Yury Norov <yury.norov@...il.com>, Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Thomas Gleixner
 <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org, kvm@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kvm: x86: simplify kvm_vector_to_index()

On 20.08.25 15:42, Yury Norov wrote:
> On Tue, Aug 19, 2025 at 04:12:11PM -0700, Sean Christopherson wrote:
>> On Sat, 19 Jul 2025 21:58:45 -0400, Yury Norov wrote:
>>> Use find_nth_bit() and make the function almost a one-liner.
>>
>> Applied to kvm-x86 misc, thanks!
>>
>> P.S. I'm amazed you could decipher the intent of the code.  Even with your
>>       patch, it took me 10+ minutes to understand the "logic".
> 
> Thanks Sean. :)
> 
>> [1/1] kvm: x86: simplify kvm_vector_to_index()
>>        https://github.com/kvm-x86/linux/commit/cc63f918a215

Is this really correct?

The original code has:

	for (i = 0; i <= mod; i++)

(note the "<=").

So it will find the (mod + 1)th bit set, so shouldn't it use

	idx = find_nth_bit(bitmap, bitmap_size, (vector % dest_vcpus) + 1);

instead?

My remark assumes that find_nth_bit(bitmap, bitmap_size, 1) will return the
same value as find_first_bit(bitmap, bitmap_size).

Juergen

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