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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Tue, 14 May 2013, Konrad Rzeszutek Wilk wrote:
> On Tue, May 14, 2013 at 02:49:50PM +0100, Stefano Stabellini wrote:
> > 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.
> 
> That looks like an API violation. We have an hypercall that
> allocates the PIRQ in the Linux, then two hypercalls in the QEMU
> layer - one to map, and the other to unmap and free.
> 
> > 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.
> 
> Sure, I am not disputing that. I think the fix in QEMU to call the
> unmap is correct.
> 
> But I am also wondering whether it makes sense to do that in the Linux
> kernel - as it does the alloc in the first place. Seems like a bit of
> duct-tape has been used to connect this plumbing together.


I admit that it is not a great interface.
I would be open to options that move the entire setup/freeing in Linux,
but keep in mind that we need to retain the pirq code in QEMU for pure
HVM guests.

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