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

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Message-ID: <aKXX1ITCwcVPrKNM@yury>
Date: Wed, 20 Aug 2025 10:12:36 -0400
From: Yury Norov <yury.norov@...il.com>
To: Juergen Gross <jgross@...e.com>
Cc: Sean Christopherson <seanjc@...gle.com>,
	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 Wed, Aug 20, 2025 at 04:01:22PM +0200, Juergen Gross wrote:
> 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).

find_nth_bit indexes those bits starting from 0, so 

find_nth_bit(bitmap, nbits, 0) == find_first_bit(bitmap, nbits)
find_nth_bit(bitmap, nbits, 1) == find_next_bit(bitmap, nbits,
                                        find_first_bit(bitmap, nbits))

And so on. Check test_find_nth_bit() for the examples.

Also, bitmap_size has a different meaning, so let's refer 'nbits'
instead.