linux-kernel - Re: [PATCH] PCI/MSI: Don't touch MSI bits when the PCI device is disconnected

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Message-ID: <5c7aa51ff9294b1aab51a3bda2416094@ausx13mps321.AMER.DELL.COM>
Date:   Tue, 18 Sep 2018 13:07:58 +0000
From:   <Alex_Gagniuc@...lteam.com>
To:     <helgaas@...nel.org>, <mr.nuke.me@...il.com>
Cc:     <linux-pci@...r.kernel.org>, <bhelgaas@...gle.com>,
        <keith.busch@...el.com>, <Austin.Bolen@...l.com>,
        <Shyam.Iyer@...l.com>, <Narendra.K@...l.com>,
        <Stuart.Hayes@...l.com>, <lukas@...ner.de>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] PCI/MSI: Don't touch MSI bits when the PCI device is
 disconnected

On 9/12/2018 4:28 PM, Bjorn Helgaas wrote:
> On Mon, Jul 30, 2018 at 04:21:44PM -0500, Alexandru Gagniuc wrote:
>> When a PCI device is gone, we don't want to send IO to it if we can
>> avoid it. We expose functionality via the irq_chip structure. As
>> users of that structure may not know about the underlying PCI device,
>> it's our responsibility to guard against removed devices.
> 
> I'm pretty ambivalent about pci_dev_is_disconnected() in general, but
> I think I'll take this, given a couple minor changelog clarifications:
> 
>> irq_write_msi_msg is already guarded. pci_msi_(un)mask_irq are not.
>> Guard them for completeness.
> 
> By the irq_write_msi_msg() guard, I guess you mean this path:
> 
>    pci_msi_domain_write_msg       # irq_chip.irq_write_msi_msg
>      __pci_write_msi_msg
>        if (dev->current_state != PCI_D0 || pci_dev_is_disconnected(dev))
>          /* don't touch */

Yes!

> pci_msi_(un)mask_irq() may be irq_chip.irq_mask, .irq_unmask, etc
> pointers.  So these are parallel because they're all irq_chip function
> pointers, but the changelog isn't (yet) parallel because it uses the
> irq_chip pointer name for .irq_write_msi_msg but not for mask/unmask

Good catch! I'll get this corrected.


>> For example, surprise removal of a PCIe device triggers teardown. This
>> touches the irq_chips ops some point to disable the interrupts. I/O
>> generated here can crash the system on machines with buggy firmware.
>> Not triggering the IO in the first place eliminates the problem.
> 
> It doesn't eliminate the problem completely because .irq_mask() and
> .irq_unmask() may be called for reasons other than surprise removal,
> and if a surprise removal happens after the pci_dev_is_disconnected()
> check but before the readl(), we will still generate I/O to a device
> that's gone.  I'd be OK if you said it "reduces" the problem.

That sounds reasonable.

> One reason I'm ambivalent about pci_dev_is_disconnected() is that in
> cases like this, it turns a reproducible problem into a very
> hard-to-reproduce problem, which reduces the likelihood that the buggy
> firmware will be fixed.

If it manages to turn this into 99.999% territory, I'll be much happier. 
I'd love to give you an academically correct solution, but I just don't 
see how, given how firmware-first philosophy is written.

> Do you have information about known platforms with this buggy firmware
> and the signature of the crash?  If you do, it's always nice to be
> able to connect a patch with the user-visible problem it fixes.

 From what I've heard, it won't be fixed. The number of changes needed 
would require re-qualifying the firmware. I'm told that's very hard to 
do on platforms that are shipping. I can reword this to say 
"firmware-first" instead of "buggy" since they are mostly synonymous.

Alex

>> Signed-off-by: Alexandru Gagniuc <mr.nuke.me@...il.com>
>> ---
>>
>> There's another patch by Lukas Wunner that is needed (not yet published)
>> in order to fully block IO on SURPRISE!!! removal. The existing code only
>> sets the PCI_DEV_DISCONNECTED bit in an unreasonably narrow set of
>> circumstances. Lukas' patch fixes that.
>>
>> However, this change is otherwise fully independent, and enjoy!
>>
>>   drivers/pci/msi.c | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/drivers/pci/msi.c b/drivers/pci/msi.c
>> index 4d88afdfc843..5f47b5cb0401 100644
>> --- a/drivers/pci/msi.c
>> +++ b/drivers/pci/msi.c
>> @@ -227,6 +227,9 @@ static void msi_set_mask_bit(struct irq_data *data, u32 flag)
>>   {
>>   	struct msi_desc *desc = irq_data_get_msi_desc(data);
>>   
>> +	if (pci_dev_is_disconnected(msi_desc_to_pci_dev(desc)))
>> +		return;
>> +
>>   	if (desc->msi_attrib.is_msix) {
>>   		msix_mask_irq(desc, flag);
>>   		readl(desc->mask_base);		/* Flush write to device */
>> -- 
>> 2.17.1
>>
>