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

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Message-ID: <20150130145631.GB27414@potion.redhat.com>
Date:	Fri, 30 Jan 2015 15:56:32 +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 10:38+0100, Paolo Bonzini:
> On 29/01/2015 22:48, Radim Krčmář wrote:
> > +		if (hweight8(new->mode) != 1)
> 
> Better (more optimized):
> 
> 	if (new->mode & (new->mode - 1))

True, hweight needs to have X86_FEATURE_POPCNT to be efficient ...

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

> 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?

> > +		if (hweight8(map->mode) != 1) {
> > +			/* Not deliverable with optimized map. */
> > +			ret = false;
> > +			goto out;
> > +		}
> 
> Put this before the computation of cid and mda.  The cid and mda are all
> wrong with a mixed map, and the result of the "if" before is influenced
> by the wrong cid.  Fixed by patch 8, but better get it right here.

Will do,

thanks.
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