Re: [virtio-comment] comment on pci section

[Rusty Russell <rusty@au1.ibm.com>]

"Michael S. Tsirkin" <mst@redhat.com> writes:
> On Wed, Jan 29, 2014 at 04:38:58PM +1030, Rusty Russell wrote:
>> "Michael S. Tsirkin" <mst@redhat.com> writes:
>> > Two comments received from a co-worker:
>> >
>> > 3.1 "The device MUST NOT consume buffers before DRIVER_OK ..." - I
>> > personally
>> > would like to see also something like:  "device must not generate
>> > interrupts if
>> > DRIVER_OK is not set” to better handle power management transitions.
>> 
>> That addition seems to make sense, but I don't understand the reference
>> to power management transitions?
>
> Well - ATM we do things like suspend by device reset and re-initialization,
> right? I'm guessing during e.g. resume it's important not to get
> interrupts too early. I'll check.
>
> Hmm actually we have a chapter about initialization but not cleanup.
> Might be a good addition.

Maybe, but it would only need to say to reset the device and you can
remove buffers safely.

Looking at the current Linux block driver, it drives the device before
setting DRIVER_OK (add_disk will do this to scan partitions).  Should we
make the rule that we can driver the device after FEATURES_OK instead?
In which case, perhaps we should just combine DRIVER_OK and FEATURES_OK
into one bit.

> Also, I note that QEMU implemented master abort bit by
> clearing DRIVER_OK - AFAIK useful to stop device in a
> standard way if driver malfunctions.
>
> Also with QEMU you can clear DRIVER_OK and this stops the device. I
> wonder if someone uses this.

The host should not change status bits, they should use a different
field.  This isn't explicitly stated in the spec, but it's a bit weird.

>> > 4.1.3.1.1.
>> > Must clarify that all devices MUST support all cfg_type values 1-5.
>> > This way BIOS can limit type 5 exclusively and drivers don't
>> > need to add support for type 5.
>> 
>> The word "support" here is unclear to me.  Are you saying that a device
>> MUST offer a VIRTIO_PCI_CAP_PCI_CFG value?  Something like:
>> 
>>         Drivers MUST offer at least one capability of each type.
>
> Exactly. Except

?

Cheers,
Rusty.