linux-kernel - Re: [PATCH v4 3/6] KVM: X86: Use APIC_DEST_* macros properly in kvm_lapic_irq.dest

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Date:   Tue, 3 Dec 2019 17:15:19 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Nitesh Narayan Lal <nitesh@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH v4 3/6] KVM: X86: Use APIC_DEST_* macros properly in
 kvm_lapic_irq.dest_mode

On Tue, Dec 03, 2019 at 02:07:52PM -0800, Sean Christopherson wrote:
> On Tue, Dec 03, 2019 at 11:59:00AM -0500, Peter Xu wrote:
> > We were using either APIC_DEST_PHYSICAL|APIC_DEST_LOGICAL or 0|1 to
> > fill in kvm_lapic_irq.dest_mode.  It's fine only because in most cases
> > when we check against dest_mode it's against APIC_DEST_PHYSICAL (which
> > equals to 0).  However, that's not consistent.  We'll have problem
> > when we want to start checking against APIC_DEST_LOGICAL, which does
> > not equals to 1.
> > 
> > This patch firstly introduces kvm_lapic_irq_dest_mode() helper to take
> > any boolean of destination mode and return the APIC_DEST_* macro.
> > Then, it replaces the 0|1 settings of irq.dest_mode with the helper.
> > 
> > Signed-off-by: Peter Xu <peterx@...hat.com>
> > ---
> >  arch/x86/include/asm/kvm_host.h | 5 +++++
> >  arch/x86/kvm/ioapic.c           | 9 ++++++---
> >  arch/x86/kvm/irq_comm.c         | 7 ++++---
> >  arch/x86/kvm/x86.c              | 2 +-
> >  4 files changed, 16 insertions(+), 7 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index b79cd6aa4075..f815c97b1b57 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -1022,6 +1022,11 @@ struct kvm_lapic_irq {
> >  	bool msi_redir_hint;
> >  };
> >  
> > +static inline u16 kvm_lapic_irq_dest_mode(bool dest_mode)
> > +{
> > +	return dest_mode ? APIC_DEST_LOGICAL : APIC_DEST_PHYSICAL;
> 
> IMO this belongs in ioapic.c as it's specifically provided for converting
> an I/O APIC redirection entry into a local APIC destination mode.  Without
> the I/O APIC context, %true==APIC_DEST_LOGICAL looks like a completely
> arbitrary decision.  And if it's in ioapic.c, it can take the union
> of a bool, which avoids the casting and shortens the callers.  E.g.:
> 
> static u64 ioapic_to_lapic_dest_mode(union kvm_ioapic_redirect_entry *e)
> {
> 	return e->fields.dest_mode ?  APIC_DEST_LOGICAL : APIC_DEST_PHYSICAL;
> }
> 
> The other option would be to use the same approach as delivery_mode and
> open code the shift.

It's also used for MSI address encodings, please see below [1].

The thing is that no matter how external protocols define destination
mode, it's always a boolean no matter where it resides, or at which bit.

Thanks,

[...]

> > @@ -114,7 +114,8 @@ void kvm_set_msi_irq(struct kvm *kvm, struct kvm_kernel_irq_routing_entry *e,
> >  		irq->dest_id |= MSI_ADDR_EXT_DEST_ID(e->msi.address_hi);
> >  	irq->vector = (e->msi.data &
> >  			MSI_DATA_VECTOR_MASK) >> MSI_DATA_VECTOR_SHIFT;
> > -	irq->dest_mode = (1 << MSI_ADDR_DEST_MODE_SHIFT) & e->msi.address_lo;
> > +	irq->dest_mode = kvm_lapic_irq_dest_mode(
> > +	    !!((1 << MSI_ADDR_DEST_MODE_SHIFT) & e->msi.address_lo));

[1]

> >  	irq->trig_mode = (1 << MSI_DATA_TRIGGER_SHIFT) & e->msi.data;
> >  	irq->delivery_mode = e->msi.data & 0x700;
> >  	irq->msi_redir_hint = ((e->msi.address_lo

-- 
Peter Xu