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

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Date:   Wed, 04 Jan 2023 12:08:38 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     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 Thu, 2022-12-29 at 10:27 +0200, 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:
> > > Do not modify AVIC's logical ID table if the logical ID portion of the
> > > LDR is not a power-of-2, i.e. if the LDR has multiple bits set.  Taking
> > > only the first bit means that KVM will fail to match MDAs that intersect
> > > with "higher" bits in the "ID"
> > > 
> > > The "ID" acts as a bitmap, but is referred to as an ID because theres an
> > > implicit, unenforced "requirement" that software only set one bit.  This
> > > edge case is arguably out-of-spec behavior, but KVM cleanly handles it
> > > in all other cases, e.g. the optimized logical map (and AVIC!) is also
> > > disabled in this scenario.
> > > 
> > > Refactor the code to consolidate the checks, and so that the code looks
> > > more like avic_kick_target_vcpus_fast().
> > > 
> > > Fixes: 18f40c53e10f ("svm: Add VMEXIT handlers for AVIC")
> > > Cc: Suravee Suthikulpanit <suravee.suthikulpanit@....com>
> > > Cc: Maxim Levitsky <mlevitsk@...hat.com>
> > > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > > ---
> > >  arch/x86/kvm/svm/avic.c | 30 +++++++++++++++---------------
> > >  1 file changed, 15 insertions(+), 15 deletions(-)
> > > 
> > > diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> > > index 4b6fc9d64f4d..a9e4e09f83fc 100644
> > > --- a/arch/x86/kvm/svm/avic.c
> > > +++ b/arch/x86/kvm/svm/avic.c
> > > @@ -513,26 +513,26 @@ unsigned long avic_vcpu_get_apicv_inhibit_reasons(struct kvm_vcpu *vcpu)
> > >  static u32 *avic_get_logical_id_entry(struct kvm_vcpu *vcpu, u32 ldr, bool flat)
> > >  {
> > >  	struct kvm_svm *kvm_svm = to_kvm_svm(vcpu->kvm);
> > > -	int index;
> > >  	u32 *logical_apic_id_table;
> > > -	int dlid = GET_APIC_LOGICAL_ID(ldr);
> > > +	u32 cluster, index;
> > >  
> > > -	if (!dlid)
> > > -		return NULL;
> > > +	ldr = GET_APIC_LOGICAL_ID(ldr);
> > >  
> > > -	if (flat) { /* flat */
> > > -		index = ffs(dlid) - 1;
> > > -		if (index > 7)
> > > +	if (flat) {
> > > +		cluster = 0;
> > > +	} 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.


Sean, please don't forget to fix this isssue in the next patch series.
Best regards,
	Maxim Levitsky
> 
> Best regards,
> 	Maxim Levitsky
> 
> 
> > For future refactoring, I also suggest to rename this function to 'avic_get_logical_id_table_entry'
> > to stress the fact that it gets a pointer to the AVIC's data structure.
> > 
> > Same for 'avic_get_physical_id_entry'
> > 
> > And also while at it : the 'svm->avic_physical_id_cache', is a very misleading name,
> > 
> > It should be svm->avic_physical_id_table_entry_ptr with a comment explaining that
> > is is the pointer to physid table entry.
> > 
> > 
> > Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>
> > 
> > 
> > 
> > Best regards,
> > 	Maxim Levitsky
> 
>