linux-kernel - Re: [PATCH v4 28/32] KVM: SVM: Require logical ID to be power-of-2 for AVIC entry

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Message-ID: <ac7beb326b44a45d5561803e62b8b94cd758a45d.camel@redhat.com>
Date:   Wed, 04 Jan 2023 20:34:28 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Alejandro Jimenez <alejandro.j.jimenez@...cle.com>,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        Li RongQing <lirongqing@...du.com>
Subject: Re: [PATCH v4 28/32] KVM: SVM: Require logical ID to be power-of-2
 for AVIC entry

On Wed, 2023-01-04 at 18:02 +0000, Sean Christopherson wrote:
> On Thu, Dec 29, 2022, mlevitsk@...hat.com wrote:
> > On Fri, 2022-12-09 at 00:00 +0200, Maxim Levitsky wrote:
> > > On Sat, 2022-10-01 at 00:59 +0000, Sean Christopherson wrote:
> > > > +	} else {
> > > > +		cluster = (ldr >> 4) << 2;
> > > > +		if (cluster >= 0xf)
> > > >  			return NULL;
> > > > -	} else { /* cluster */
> > > > -		int cluster = (dlid & 0xf0) >> 4;
> > > > -		int apic = ffs(dlid & 0x0f) - 1;
> > > > -
> > > > -		if ((apic < 0) || (apic > 7) ||
> > > > -		    (cluster >= 0xf))
> > > > -			return NULL;
> > > > -		index = (cluster << 2) + apic;
> > > > +		ldr &= 0xf;
> > > >  	}
> > > > +	if (!ldr || !is_power_of_2(ldr))
> > > > +		return NULL;
> > > > +
> > > > +	index = __ffs(ldr);
> > > > +	if (WARN_ON_ONCE(index > 7))
> > > > +		return NULL;
> > > > +	index += (cluster << 2);
> > > >  
> > > >  	logical_apic_id_table = (u32 *) page_address(kvm_svm->avic_logical_id_table_page);
> > > >  
> > > 
> > > Looks good.
> > 
> > I hate to say it but this patch has a bug:
> > 
> > We have both 'cluster = (ldr >> 4) << 2' and then 'index += (cluster << 2)'
> > 
> > One of the shifts has to go.
> 
> The first shift is wrong.  The "cluster >= 0xf" check needs to be done on the actual
> cluster.  The "<< 2", a.k.a. "* 4", is specific to indexing the AVIC table.
> 
> Thanks!
> 

Yep, agree.

(I mean technically you can remove the second shift and do the check before doing the first shift, that
is why I told you that one of the shifts has to go)

Best regards,
	Maxim Levitsky