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

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Date:	Thu, 12 Mar 2015 23:35:45 -0400
From:	Bandan Das <bsd@...hat.com>
To:	Fam Zheng <famz@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>,
	linux-pci@...r.kernel.org,
	Alex Williamson <alex.williamson@...hat.com>
Subject: Re: [RFC PATCH] PCI: Disable MSI/MSI-X only if device is shutdown

Fam Zheng <famz@...hat.com> writes:

> On Thu, 03/12 23:09, 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.
>> >> >
>> >> > 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.
>
> I don't understand, do you mean that no reset will be done before more
> operations on the device?

I meant that it's upto the individual driver, pci will take the same path
as a regular reboot.

>> 
>> > 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.
>
> This virtio-scsi case shows us that intermediate state is bad, so I still think
> we should call pci_msix_shutdown conditionally.
>
> Fam
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