Re: [Xen-devel] [RFC 06/19] xen/arm: Implement hypercall PHYSDEVOP_map_pirq

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 19 Jun 2014, Julien Grall wrote:
> On 06/18/2014 08:24 PM, Stefano Stabellini wrote:
> >>  /*
> >> diff --git a/xen/arch/arm/vgic.c b/xen/arch/arm/vgic.c
> >> index e451324..c18b2ca 100644
> >> --- a/xen/arch/arm/vgic.c
> >> +++ b/xen/arch/arm/vgic.c
> >> @@ -82,10 +82,7 @@ int domain_vgic_init(struct domain *d)
> >>      /* Currently nr_lines in vgic and gic doesn't have the same meanings
> >>       * Here nr_lines = number of SPIs
> >>       */
> >> -    if ( is_hardware_domain(d) )
> >> -        d->arch.vgic.nr_lines = gic_number_lines() - 32;
> >> -    else
> >> -        d->arch.vgic.nr_lines = 0; /* We don't need SPIs for the guest */
> >> +    d->arch.vgic.nr_lines = gic_number_lines() - 32;
> >>  
> >>      d->arch.vgic.shared_irqs =
> >>          xzalloc_array(struct vgic_irq_rank, DOMAIN_NR_RANKS(d));
> > 
> > I see what you mean about virq != pirq.
> > 
> > It seems to me that setting d->arch.vgic.nr_lines = gic_number_lines() -
> > 32 for the hardware domain is OK, but it is really a waste for the
> > others. We could find a way to pass down the info about how many SPIs we
> > need from libxl. Or we could delay the vgic allocations until the first
> > SPI is assigned to the domU.
> 
> I gave a check on both midway and the versatile express and there is
> about 200 lines.
> 
> It make the overhead of less than 8K per domain. Which is not too bad.
> 
> If the host really support 1024 IRQs that would make an overhead of ~32K.
> 
> > Similarly to the MMIO hole sizing, I don't think that it would be a
> > requirement for this patch series but it is something to keep in mind.
> 
> Handling virq != pirq will be more complex as we need to take into
> account of the hotplug solution.
> 
> The vgic has a register which provide the number of lines, I suspect
> this number can't grow up while the guest is running.

Of course not. But keep in mind that for non-PCI passthrough we would be
fully aware of all the assigned interrupts before starting the VM.

PCI passthrough and MSI-X are the issue because there can be many MSI
per device and the device can be hotplugged into the guest.

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