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From:   Dexuan Cui <decui@microsoft.com>
To:     Bjorn Helgaas <helgaas@kernel.org>
CC:     'Bjorn Helgaas' <bhelgaas@google.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        'Lorenzo Pieralisi' <lorenzo.pieralisi@arm.com>,
        "'linux-pci@vger.kernel.org'" <linux-pci@vger.kernel.org>,
        KY Srinivasan <kys@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        "'olaf@aepfle.de'" <olaf@aepfle.de>,
        "'apw@canonical.com'" <apw@canonical.com>,
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        <driverdev-devel@linuxdriverproject.org>,
        "'vkuznets@redhat.com'" <vkuznets@redhat.com>,
        "'marcelo.cerri@canonical.com'" <marcelo.cerri@canonical.com>
Subject: RE: [PATCH] PCI: hv: Fix a __local_bh_enable_ip warning in
 hv_compose_msi_msg()
Thread-Topic: [PATCH] PCI: hv: Fix a __local_bh_enable_ip warning in
 hv_compose_msi_msg()
Thread-Index: AdPyKucLHxE0xn3+RTSKXedSJeHHcgCNtVtQAmSYQFABWDo6wAADvceAAAAiUoA=
Date:   Wed, 13 Jun 2018 22:50:05 +0000
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> From: Bjorn Helgaas <helgaas@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?
>=20
> 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!
=20
> You might, however, fix the commit log:
>=20
>   This is not an issue because hv_pci_onchannelcallback() is not slow,
>   and it not a hot path.
>=20
> 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.
=20
> 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()?
>=20
> Bjorn
Actually in my original internal version of the patch, I did use=20
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()/enab=
le().

However, recently I found that if we enable CONFIG_PROVE_LOCKING=3Dy,
the local_bh_enable() can trigger a warning because the function=20
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