linux-kernel - Re: [PATCH 7/8] KVM: x86: avoid logical

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Message-ID: <20150130170909.GE27414@potion.redhat.com>
Date:	Fri, 30 Jan 2015 18:09:10 +0100
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Nadav Amit <namit@...technion.ac.il>,
	Gleb Natapov <gleb@...nel.org>
Subject: Re: [PATCH 7/8] KVM: x86: avoid logical_map when it is invalid

2015-01-30 16:10+0100, Paolo Bonzini:
> On 30/01/2015 15:56, Radim Krčmář wrote:
> > Do you know of a difference with it?
> >   new->mode & (new->mode - 1)   |  hweight8(new->mode) != 1
> >     lea    -0x1(%rax),%edi      |    popcnt %edi,%eax
> >     test   %eax,%edi            |    cmp    $1,%eax
> 
> x & (x - 1) is really hweight8(new->mode) > 1.  So if new->mode == 0 it
> would have a different result.

(I was thinking if execution profile of those two instructions isn't
 vastly different.

 ">" in this check works too ... later it seemed like it was pushing
 lucky coincidence too far, so it ended as "!=" :)

> >> Please add a comment to kvm_irq_delivery_to_apic_fast to explain what
> >> you are doing.
> > 
> > Would naming it kvm_apic_need_slow_delivery(), or something, be enough?
> 
> Or kvm_apic_map_valid() perhaps?

Sure, thanks.
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