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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Thu, 2014-06-19 at 13:29 +0100, Stefano Stabellini wrote:
> 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.

What's the issue here? Something to do with irqdesc->irq-pending lookup?

Seems like irqdesc needs to store the domain and virq number when the
irq is passed through. I assume it must store the dmain already.


> > 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.

Are we ruling out hotplug of such devices? (I don't have a problem with
that BTW)

> 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.

MSI(-X) AKA LPIs are in a different more dynamic number space though
(from 8192 onwards). I think for that specific case we can dynamically
do things.

The bigger issue would be the legacy INT-x interrupts (which I expect
look like SPIs), those would no doubt need exposing somehow.

Do we think it is the case that we are eventually going to need a guest
cfg option pci = 0|1? I think the answer is yes. Assinging a pci device
would cause pci=1, or you can set pci=1 to enable hotplug of pci devices
later (i.e. mmio space is reserved, INTx interrupts are assigned etc).

We already have something not totally different in the e820_host option
on x86.

Ian.


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