Re: [PATCH] PCI: don't allow "pci-phantom=" to mark real devices as phantom functions

[Jan Beulich <jbeulich@xxxxxxxx>]

On 05.05.2022 21:10, Andrew Cooper wrote:
> On 29/04/2022 14:05, Jan Beulich wrote:
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>>
>> IOMMU code mapping / unmapping devices and interrupts will misbehave if
>> a wrong command line option declared a function "phantom" when there's a
>> real device at that position. Warn about this and adjust the specified
>> stride (in the worst case ignoring the option altogether).
>>
>> Requested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>
>> --- a/xen/drivers/passthrough/pci.c
>> +++ b/xen/drivers/passthrough/pci.c
>> @@ -451,7 +451,24 @@ static struct pci_dev *alloc_pdev(struct
>>                           phantom_devs[i].slot == PCI_SLOT(devfn) &&
>>                           phantom_devs[i].stride > PCI_FUNC(devfn) )
>>                      {
>> -                        pdev->phantom_stride = phantom_devs[i].stride;
>> +                        pci_sbdf_t sbdf = pdev->sbdf;
>> +                        unsigned int stride = phantom_devs[i].stride;
>> +
>> +                        while ( (sbdf.fn += stride) > PCI_FUNC(devfn) )
> 
> I'm fairly sure this doesn't do what you want it to.
> 
> .fn is a 3 bit bitfield, meaning the += will be truncated before the
> compare.

And this is precisely what I'm after: I want to stop once the value
has wrapped.

>> +                        {
>> +                            if ( pci_conf_read16(sbdf, PCI_VENDOR_ID) == 
>> 0xffff &&
>> +                                 pci_conf_read16(sbdf, PCI_DEVICE_ID) == 
>> 0xffff )
>> +                                continue;
>> +                            stride <<= 1;
>> +                            printk(XENLOG_WARNING
>> +                                   "%pp looks to be a real device; bumping 
>> %04x:%02x:%02x stride to %u\n",
>> +                                   &sbdf, phantom_devs[i].seg,
>> +                                   phantom_devs[i].bus, 
>> phantom_devs[i].slot,
>> +                                   stride);
>> +                            sbdf = pdev->sbdf;
>> +                        }
>> +                        if ( PCI_FUNC(stride) )
> 
> This is an obfuscated way of writing stride < 8.

And intentionally so, matching a few other similar instances elsewhere.
An open-coded 8 here doesn't really make clear where that 8 would be
coming from. The use of PCI_FUNC(), otoh, documents what's meant.

> Given the printk(), if we actually find an 8-function device, what gets
> printed (AFAICT) will be "bumping to 8" when in fact we mean "totally
> ignoring the option".  I think this really wants an else clause.

Yes, "bumping to 8" is what is being printed in that case. I did
realize the slight anomaly when writing the code and I observed
(verified) it also in testing. But I didn't see a good reason for an
"else" here - 8 being mentioned in the log message is clear enough
for anyone vaguely understanding phantom functions. But if you strongly
think we need to make the code yet larger and indentation yet
unhelpfully deeper, then I will (begrudgingly) do what you ask for. But
please explicitly confirm.

Jan