[PATCH] PCI: hv: Fix a __local_bh_enable_ip warning in hv_compose_msi_msg()

[Dexuan Cui]

> From: Bjorn Helgaas <helgaas at kernel.org>
> Sent: Wednesday, June 13, 2018 15:15
> > ...
> > It looks Lorenzo's pci.git tree has not been updated for 3+ weeks.
> > I guess Lorenzo may be on vacation.
> >
> > @Bjorn, can this patch go through your tree?
> > Should I resubmit it?
> 
> No need to resubmit it, Lorenzo has been out for a bit, but I'm sure
> he'll pick this up as he catches up.
OK, I see. Thanks!
 
> You might, however, fix the commit log:
> 
>   This is not an issue because hv_pci_onchannelcallback() is not slow,
>   and it not a hot path.
> 
> This has at least one typo (I think you mean "and *is* not a hot
> path").
Sorry -- yes, it's a typo. I hope Lorenzo can help to fix this, or I can
resubmit it if Lorenzo or you want me to do it.
 
> I also don't understand the sentence as a whole because the
> hv_pci_onchannelcallback() comment says it's called whenever the host
> sends a packet to this channel, and that *does* sound like a hot path.
Sorry for not making it clear.
The host only sends a packet into the channel of the guest when there
is a change of device configuration (i.e. hot add or remove a device), or
the host is responding to the guest's request.

The change of device configuration is only triggered on-demand by the
administrator on the host, and the guest's requests are one-off when
the device is probed.

So IMO the callback is not a hot path.

> I also don't understand the "hv_pci_onchannelcallback() is not slow"
> part.  In other words, you're saying hv_pci_onchannelcallback() is
> fast and it's not a hot path.  And apparently this has something to do
> with the difference between local_bh_disable() and local_irq_save()?
> 
> Bjorn
Actually in my original internal version of the patch, I did use 
local_irq_save/restore().

hv_pci_onchannelcallback() itself runs fast, but here since it's in a
loop (i.e. the while (!try_wait_for_completion(&comp.comp_pkt.host_event)
loop), IIRC I was asked if I really need local_irq_save/restore(),
and I answered "not really", so later I switched to local_bh_disable()/enable().

However, recently I found that if we enable CONFIG_PROVE_LOCKING=y,
the local_bh_enable() can trigger a warning because the function 
hv_compose_msi_msg() can be called with local IRQs disabled (BTW,
hv_compose_msi_msg() can also be called with local IRQS enabled in
another code path):

IRQs not enabled as expected
  WARNING: CPU: 0 PID: 408 at kernel/softirq.c:162 __local_bh_enable_ip

Despite the warning, the code itself can still work correctly, but IMO we'd
better switch back to local_irq_save/restore(), and hence I made the patch.

I hope the explanation sounds reasonable. :-)

Thanks,
-- Dexuan