Re: [Xen-devel] [PATCH 11/28] ARM: GICv3: forward pending LPIs to guests

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 15 Feb 2017, Julien Grall wrote:
> Hi Stefano,
> 
> On 14/02/17 21:00, Stefano Stabellini wrote:
> > On Mon, 30 Jan 2017, Andre Przywara wrote:
> > > +/*
> > > + * Handle incoming LPIs, which are a bit special, because they are
> > > potentially
> > > + * numerous and also only get injected into guests. Treat them specially
> > > here,
> > > + * by just looking up their target vCPU and virtual LPI number and hand
> > > it
> > > + * over to the injection function.
> > > + */
> > > +void do_LPI(unsigned int lpi)
> > > +{
> > > +    struct domain *d;
> > > +    union host_lpi *hlpip, hlpi;
> > > +    struct vcpu *vcpu;
> > > +
> > > +    WRITE_SYSREG32(lpi, ICC_EOIR1_EL1);
> > > +
> > > +    hlpip = gic_get_host_lpi(lpi);
> > > +    if ( !hlpip )
> > > +        return;
> > > +
> > > +    hlpi.data = read_u64_atomic(&hlpip->data);
> > > +
> > > +    /* We may have mapped more host LPIs than the guest actually asked
> > > for. */
> > > +    if ( !hlpi.virt_lpi )
> > > +        return;
> > > +
> > > +    d = get_domain_by_id(hlpi.dom_id);
> > > +    if ( !d )
> > > +        return;
> > > +
> > > +    if ( hlpi.vcpu_id >= d->max_vcpus )
> > > +    {
> > > +        put_domain(d);
> > > +        return;
> > > +    }
> > > +
> > > +    vcpu = d->vcpu[hlpi.vcpu_id];
> > > +
> > > +    put_domain(d);
> > > +
> > > +    vgic_vcpu_inject_irq(vcpu, hlpi.virt_lpi);
> > 
> > put_domain should be here
> 
> Why? I don't even understand why we would need to take a reference on the
> domain for LPIs. Would not it be enough to use rcu_lock_domain_by_id here?

I think that rcu_lock_domain_by_id would also work, but similarly we
would need to call rcu_unlock here.

To be honest, I don't know exactly in which cases get_domain should be
used instead of rcu_lock_domain_by_id.

CC'ing the x86 guys that might know the answer.

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