Re: [PATCH] xen: reuse the same pirq allocated when driver load first time

[Stefano Stabellini <stefano.stabellini@eu.citrix.com>]

On Mon, 13 May 2013, Konrad Rzeszutek Wilk wrote:
> On Mon, May 13, 2013 at 06:24:46PM +0100, Stefano Stabellini wrote:
> > On Mon, 13 May 2013, Konrad Rzeszutek Wilk wrote:
> > > On Mon, May 13, 2013 at 03:50:52PM +0100, Stefano Stabellini wrote:
> > > > On Mon, 13 May 2013, Konrad Rzeszutek Wilk wrote:
> > > > > On Mon, May 13, 2013 at 12:06:43PM +0100, Stefano Stabellini wrote:
> > > > > > On Fri, 10 May 2013, Konrad Rzeszutek Wilk wrote:
> > > > > > > On Wed, May 08, 2013 at 04:18:24PM +0800, Zhenzhong Duan wrote:
> > > > > > > > When driver load and unload in a loop, pirq will exhaust finally.
> > > > > > > > Try to use the same pirq which was already mapped and binded at first time
> > > > > > > 
> > > > > > > So what happens if I unload and reload two drivers in random order?
> > > > > > > 
> > > > > > > > when driver loaded.
> > > > > > > > 
> > > > > > > > Read pirq from msix entry and test if data is XEN_PIRQ_MSI_DATA
> > > > > > > > xen_irq_from_pirq(pirq) < 0 checking is wrong as irq will be freed
> > > > > > > > when driver unload, it's always true in second load.
> > > > > > > 
> > > > > > > If my understanding is right the issue at hand is that the caching
> > > > > > > information about the pirq disappears once the driver has been
> > > > > > > unloaded b/c the event's irq-info is removed (as the driver is
> > > > > > > unloaded and free_irq is called).
> > > > > > > 
> > > > > > > Stefano,
> > > > > > > Is there a specific write to the MSI structure that would cause the
> > > > > > > hypervisor to drop the PIRQ? Or a nice hypercall to "free" an
> > > > > > > PIRQ in usage?
> > > > > > 
> > > > > > We already have a "free PIRQ" hypercall, it's called
> > > > > > PHYSDEVOP_unmap_pirq and should be called by QEMU.
> > > > > 
> > > > > Considering that we call function that allocates (PHYSDEVOP_get_free_pirq)
> > > > > it in the Linux kernel (and not in QEMU), perhaps that should be done in the
> > > > > Linux kernel as part of xen_destroy_irq()? Or would that confuse QEMU?
> > > > 
> > > > I think it would confuse QEMU. It is probably better to let the unmap
> > > > being handled by it.
> > > > 
> > > > 
> > > > > It looks like QEMU only does that hypercall (via xc_physdev_unmap_pirq)
> > > > > unregister_real_device which is only called during pci unplug?
> > > > 
> > > > You are right! I would think that this behaviour is erroneous unless it
> > > > was done on purpose to avoid allocating MSIs twice.
> > > > If that is the case we would need to do something similar in Linux too.
> > > > 
> > > > I think that the issue is the mismatch between QEMU's and Linux's
> > > > behaviours: either both should be allocating MSIs once, or they should
> > > > both be allocating and deallocating MSIs every time the driver is loaded
> > > > and unloaded.
> > > 
> > > Right. But we also have the scenario that QEMU and Linux are going to 
> > > be out of sync. So we need fixes in both places - I think.
> > 
> > QEMU is the owner of the pirq, in fact it is the one that creates and
> > destroys the mapping. I think that the right place to fix this problem
> > is in QEMU, the ABI would be much cleaner as a result. As a side effect
> > we don't need to make any changes in Linux.
> 
> You do. You need to remove the PHYSDEVOP_get_free_pirq call in that case.
 
PHYSDEVOP_get_free_pirq needs to stay, because Linux needs to know the
pirq that QEMU is going to use.

However I would let QEMU handle the mapping (it already does that in
pt_msi_setup calling xc_physdev_map_pirq_msi) and unmapping (that is
done by calling xc_domain_unbind_msi_irq from pt_msi_disable).
I think the problem is that pt_msi_disable is only called on
unregister_real_device and pt_reset_interrupt_and_io_mapping, not when
the guest disables MSIs.