Linux Virtualization - Sep 2015 - [PATCH v7] pci: quirk to skip msi disable on shutdown

On Mon, Sep 21, 2015 at 01:21:47PM -0500, Bjorn Helgaas
wrote:
> On Sun, Sep 06, 2015 at 06:32:35PM +0300, Michael S. Tsirkin wrote:
> > On some hypervisors, virtio devices tend to generate spurious
interrupts
> > when switching between MSI and non-MSI mode.  Normally, either MSI or
> > non-MSI is used and all is well, but during shutdown, linux disables
MSI
> > which then causes an "irq %d: nobody cared" message, with
irq being
> > subsequently disabled.
> 
> My understanding is:
> 
>   Linux disables MSI/MSI-X during device shutdown.  If the device
>   signals an interrupt after that, it may use INTx.
> 
> This INTx interrupt is not necessarily spurious.  Using INTx to signal an
> interrupt that occurs when MSI is disabled seems like reasonable behavior
> for any PCI device.
> And it doesn't seem related to switching between MSI and non-MSI mode.
> Yes, the INTx happens *after* disabling MSI, but it is not at all
> *because* we disabled MSI.  So I wouldn't say "they generate
spurious
> interrupts when switching between MSI and non-MSI."
> 
> Why doesn't virtio-pci just register an INTx handler in addition to an
MSI
> handler?
> 
> Bjorn
The handler causes an expensive exit to the hypervisor,
and the INTx lines are shared with other devices.

Seems silly to slow them down just so we can do something
that triggers the device bug.  The bus master is disabled by that time,
if linux can just desist from touching MSI enable device won't
send either INTx (because MSI is on) or MSI
(because bus master is on) and all will be well.

-- 
MST

On Mon, Sep 21, 2015 at 10:42:13PM +0300, Michael S. Tsirkin
wrote:
> On Mon, Sep 21, 2015 at 01:21:47PM -0500, Bjorn Helgaas wrote:
> > On Sun, Sep 06, 2015 at 06:32:35PM +0300, Michael S. Tsirkin wrote:
> > > On some hypervisors, virtio devices tend to generate spurious
interrupts
> > > when switching between MSI and non-MSI mode.  Normally, either
MSI or
> > > non-MSI is used and all is well, but during shutdown, linux
disables MSI
> > > which then causes an "irq %d: nobody cared" message,
with irq being
> > > subsequently disabled.
> > 
> > My understanding is:
> > 
> >   Linux disables MSI/MSI-X during device shutdown.  If the device
> >   signals an interrupt after that, it may use INTx.
> > 
> > This INTx interrupt is not necessarily spurious.  Using INTx to signal
an
> > interrupt that occurs when MSI is disabled seems like reasonable
behavior
> > for any PCI device.
> > And it doesn't seem related to switching between MSI and non-MSI
mode.
> > Yes, the INTx happens *after* disabling MSI, but it is not at all
> > *because* we disabled MSI.  So I wouldn't say "they generate
spurious
> > interrupts when switching between MSI and non-MSI."
> > 
> > Why doesn't virtio-pci just register an INTx handler in addition
to an MSI
> > handler?
> 
> The handler causes an expensive exit to the hypervisor,
> and the INTx lines are shared with other devices.
Do we care?  Is this a performance path?  I thought we were in a kexec
shutdown path.
> Seems silly to slow them down just so we can do something
> that triggers the device bug.  The bus master is disabled by that time,
> if linux can just desist from touching MSI enable device won't
> send either INTx (because MSI is on) or MSI
> (because bus master is on) and all will be well.
It would also be silly to put special-purpose code in the PCI core
if there's a reasonable way to handle this in a driver.

Can you describe exactly what the device bug is?  Apparently you're
saying that if we shut down MSI, it triggers the bug?  And I guess
you're talking about a virtio device as implemented in qemu or other
hypervisors?

If we leave MSI enabled (as your patch does), then the device has MSI
enabled and Bus Master disabled.  I can see these possibilities:

  1) the device never recognizes an interrupt condition
? 2) the device sets the pending bit but doesn't issue the MSI write,
     so the OS doesn't see the interrupt unless it polls for it
? 3) the device signals MSI and we still have?an MSI handler
     registered, so we silently handle it
  4) the device signals INTx

You seem to suggest that if we leave MSI enabled (as your patch does),
we're in case 1.  But I doubt that disabling MSI causes the device to
interrupt.

Case 2 seems more likely to me: the device recognized an interrupt
condition, e.g., an event occurred, and the OS simply doesn't see the
interrupt because the device can't issue the MSI message.

Case 3 does seem like it would be a device bug, because the device
shouldn't do an MSI write when Bus Master is disabled.  I don't see
this case mentioned explicitly in the PCI spec, but PCIe r3.0 spec sec
7.5.1.1 does make it clear that disabling Bus Master also disables MSI
messages.

I don't know whether case 4 would be legal or not.  But apparently it
doesn't happen with the virtio device anyway, so it's not really a
concern here.

Bjorn

On Mon, Sep 21, 2015 at 05:10:43PM -0500, Bjorn Helgaas
wrote:
> On Mon, Sep 21, 2015 at 10:42:13PM +0300, Michael S. Tsirkin wrote:
> > On Mon, Sep 21, 2015 at 01:21:47PM -0500, Bjorn Helgaas wrote:
> > > On Sun, Sep 06, 2015 at 06:32:35PM +0300, Michael S. Tsirkin
wrote:
> > > > On some hypervisors, virtio devices tend to generate
spurious interrupts
> > > > when switching between MSI and non-MSI mode.  Normally,
either MSI or
> > > > non-MSI is used and all is well, but during shutdown, linux
disables MSI
> > > > which then causes an "irq %d: nobody cared"
message, with irq being
> > > > subsequently disabled.
> > > 
> > > My understanding is:
> > > 
> > >   Linux disables MSI/MSI-X during device shutdown.  If the device
> > >   signals an interrupt after that, it may use INTx.
> > > 
> > > This INTx interrupt is not necessarily spurious.  Using INTx to
signal an
> > > interrupt that occurs when MSI is disabled seems like reasonable
behavior
> > > for any PCI device.
> > > And it doesn't seem related to switching between MSI and
non-MSI mode.
> > > Yes, the INTx happens *after* disabling MSI, but it is not at all
> > > *because* we disabled MSI.  So I wouldn't say "they
generate spurious
> > > interrupts when switching between MSI and non-MSI."
> > > 
> > > Why doesn't virtio-pci just register an INTx handler in
addition to an MSI
> > > handler?
> > 
> > The handler causes an expensive exit to the hypervisor,
> > and the INTx lines are shared with other devices.
> 
> Do we care?  Is this a performance path?  I thought we were in a kexec
> shutdown path.
Yes but the handler would always have to be registered, right?
> > Seems silly to slow them down just so we can do something
> > that triggers the device bug.  The bus master is disabled by that
time,
> > if linux can just desist from touching MSI enable device won't
> > send either INTx (because MSI is on) or MSI
> > (because bus master is on) and all will be well.
> 
> It would also be silly to put special-purpose code in the PCI core
> if there's a reasonable way to handle this in a driver.
> 
> Can you describe exactly what the device bug is?  Apparently you're
> saying that if we shut down MSI, it triggers the bug?  And I guess
> you're talking about a virtio device as implemented in qemu or other
> hypervisors?
Yes. Basically depending on an internal device state, disabling MSI
sometimes wedges it.  The most easy to debug effect is if it starts
sending INTx interrupts, for which there's no handler currently.
Full system reset always gets us out of the bad state.

> If we leave MSI enabled (as your patch does), then the device has MSI
> enabled and Bus Master disabled.  I can see these possibilities:
> 
>   1) the device never recognizes an interrupt condition
> ? 2) the device sets the pending bit but doesn't issue the MSI write,
>      so the OS doesn't see the interrupt unless it polls for it
> ? 3) the device signals MSI and we still have?an MSI handler
>      registered, so we silently handle it
>   4) the device signals INTx
> You seem to suggest that if we leave MSI enabled (as your patch does),
> we're in case 1.  But I doubt that disabling MSI causes the device to
> interrupt.
> 
> Case 2 seems more likely to me: the device recognized an interrupt
> condition, e.g., an event occurred, and the OS simply doesn't see the
> interrupt because the device can't issue the MSI message.
> 
> Case 3 does seem like it would be a device bug, because the device
> shouldn't do an MSI write when Bus Master is disabled.  I don't see
> this case mentioned explicitly in the PCI spec, but PCIe r3.0 spec sec
> 7.5.1.1 does make it clear that disabling Bus Master also disables MSI
> messages.
> 
> I don't know whether case 4 would be legal or not.  But apparently it
> doesn't happen with the virtio device anyway, so it's not really a
> concern here.
> 
> Bjorn
It's case 2. Except it doesn't actually set the pending
bit, because PCI spec has language like

	"While a vector is masked, the function is prohibited from sending the
	associated message, and the function must set the associated Pending bit"

but doesn't actually require to set pending if bus master enable
prevents sending MSI.

-- 
MST