linux-kernel - Re: [PATCH v4] vfio/pci: Propagate ACPI notifications to user-space via eventfd

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Date:   Wed, 7 Jun 2023 16:26:55 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Grzegorz Jaszczyk <jaz@...ihalf.com>
Cc:     linux-kernel@...r.kernel.org, dmy@...ihalf.com, tn@...ihalf.com,
        dbehr@...gle.com, dbehr@...omium.org, upstream@...ihalf.com,
        dtor@...gle.com, jgg@...pe.ca, kevin.tian@...el.com,
        cohuck@...hat.com, abhsahu@...dia.com, yishaih@...dia.com,
        yi.l.liu@...el.com, kvm@...r.kernel.org, libvir-list@...hat.com,
        pmorel@...ux.ibm.com, borntraeger@...ux.ibm.com,
        mjrosato@...ux.ibm.com
Subject: Re: [PATCH v4] vfio/pci: Propagate ACPI notifications to user-space
 via eventfd

On Wed, 7 Jun 2023 22:22:12 +0200
Grzegorz Jaszczyk <jaz@...ihalf.com> wrote:
> > >
> > > Can we drop the NTFY and just use VFIO_PCI_ACPI_IRQ_INDEX?  
> >
> > ACPI_IRQ at first glance could be confused with SCI, which is e.g.
> > registered as "acpi" irq seen in /proc/interrupts, maybe it is worth
> > keeping NTFY here to emphasise the "Notify" part?  
> 
> Please let me know if you prefer VFIO_PCI_ACPI_IRQ_INDEX or
> VFIO_PCI_ACPI_NTFY_IRQ_INDEX taking into account the above.

This is a device level ACPI interrupt, so it doesn't seem like it would
be confused with SCI.  What other ACPI related interrupts would a
device have?  I'm still partial to dropping the NTFY but if you're
attached to it, let's not abbreviate it, make it NOTIFY and do the same
for function names.

...
> > > > +     } else if (flags & VFIO_IRQ_SET_DATA_BOOL) {
> > > > +             u32 notification_val;
> > > > +
> > > > +             if (!count)
> > > > +                     return -EINVAL;
> > > > +
> > > > +             notification_val = *(u32 *)data;  
> > >
> > > DATA_BOOL is defined as a u8, and of course also as a bool, so we
> > > expect only zero/non-zero.  I think a valid interpretation would be any
> > > non-zero value generates a device check notification value.  
> >
> > Maybe it would be helpful and ease testing if we could use u8 as a
> > notification value placeholder so it would be more flexible?
> > Notification values from 0x80 to 0xBF are device-specific, 0xC0 and
> > above are reserved for definition by hardware vendors for hardware
> > specific notifications and BTW in practice I didn't see notification
> > values that do not fit in u8 but even if exist we can limit to u8 and
> > gain some flexibility anyway. Please let me know what you think.  
> 
> Does the above seem ok for you?

The data type is only a u8 for practicality, it's still labeled as a
bool which suggests it's interpreted as either zero or non-zero.  We
also need to reconcile DATA_NONE, which should trigger the interrupt,
but with an implicit notification value.  I see the utility in what
you're proposing, but it logically implies an extension of the SET_IRQS
ioctl for a new data type which has hardly any practical value.  Thanks,

Alex