linux-kernel - Re: [RFC PATCH] PCI: Disable MSI/MSI-X only if device is shutdown

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Date:	Fri, 13 Mar 2015 12:53:26 +0800
From:	Fam Zheng <famz@...hat.com>
To:	Alex Williamson <alex.williamson@...hat.com>
Cc:	Bandan Das <bsd@...hat.com>, linux-kernel@...r.kernel.org,
	Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org
Subject: Re: [RFC PATCH] PCI: Disable MSI/MSI-X only if device is shutdown

On Thu, 03/12 22:13, Alex Williamson wrote:
> On Thu, 2015-03-12 at 23:09 -0400, Bandan Das wrote:
> > Ccing Alex, he can probably confirm if my understanding is indeed correct.
> > 
> > Fam Zheng <famz@...hat.com> writes:
> > 
> > > On Thu, 03/12 19:56, Bandan Das wrote:
> > >> Hi Fam,
> > >> 
> > >> Fam Zheng <famz@...hat.com> writes:
> > >> 
> > >> > If the device doesn't support shutdown, disabling interrupts may cause
> > >> > trouble. For example, virtio-scsi-pci doesn't implement shutdown, and
> > >> > after we disable MSI-X, futher notifications from device will be
> > >> > delivered to IRQ, which is unexpected. This IRQ will not be cleared, and
> > >> > may prevent us from making progress, by keep triggering interrupts.
> 
> Won't it be disabled as a spurious interrupt if it's retriggering that
> quickly?

You're right, it will. Thanks.

Fam

> 
> > >> > Signed-off-by: Fam Zheng <famz@...hat.com>
> > >> > ---
> > >> >  drivers/pci/pci-driver.c | 7 ++++---
> > >> >  1 file changed, 4 insertions(+), 3 deletions(-)
> > >> >
> > >> > diff --git a/drivers/pci/pci-driver.c b/drivers/pci/pci-driver.c
> > >> > index 3cb2210..fb29c96 100644
> > >> > --- a/drivers/pci/pci-driver.c
> > >> > +++ b/drivers/pci/pci-driver.c
> > >> > @@ -448,10 +448,11 @@ static void pci_device_shutdown(struct device *dev)
> > >> >  
> > >> >  	pm_runtime_resume(dev);
> > >> >  
> > >> > -	if (drv && drv->shutdown)
> > >> > +	if (drv && drv->shutdown) {
> > >> >  		drv->shutdown(pci_dev);
> > >> > -	pci_msi_shutdown(pci_dev);
> > >> > -	pci_msix_shutdown(pci_dev);
> > >> > +		pci_msi_shutdown(pci_dev);
> > >> > +		pci_msix_shutdown(pci_dev);
> > >> > +	}
> > >> 
> > >> If the driver doesn't provide a shutdown method and doesn't itself
> > >> disable MSI/MSI-X, then pci_msi(x)_shutdown won't be called anymore,
> > >> no ?
> > >
> > > Right.
> > >
> > >> 
> > >> This is probably ok (but unclean) for a system shutdown, but could
> > >> cause problems for something like kexec ?
> > >
> > > Doesn't the reset in kexec clean this up during initialization?
> > 
> > I guess it would take the same path as a reboot.
> 
> If we were to kexec, the only thing I see touching MSI/X enable bits is
> pci_msi_init_pci_dev(), which unconditionally disables MSI/X as we walk
> PCI and discover devices.  Why is disabling there any different?  A new
> instance of the virtio driver hasn't yet been bound to the device to
> quiesce it.
> 
> > > Without shutdown, anything in the driver is unclean anyway, for example
> > > free_irq is not called.
> > 
> > True, And that is why the MSI/-X shutdown functions are called here
> > because pci can't always rely on the individual device drivers to do
> > the right thing, but atleast can make a best effort at cleaning up.
> 
> A concern with this patch would be that some drivers potentially don't
> implement a shutdown routine because they rely on pci-core to do this
> minimal cleanup.  I'm tempted to suggest adding a call to pci_intx() to
> disable INTx as part of the PCI-core shutdown, but that sounds like
> asking for trouble with broken legacy devices.  It sure seems a lot
> safer to make a virtio-scsi-pci shutdown function.  Thanks,
> 
> Alex
> 
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