Message-ID: <ac3eb2510812100402v58f792e3k6ebd6607495a13bf@mail.gmail.com>
Date:	Wed, 10 Dec 2008 13:02:06 +0100
From:	"Kay Sievers" <kay.sievers@...y.org>
To:	"Mark McLoughlin" <markmc@...hat.com>
Cc:	"Anthony Liguori" <aliguori@...ibm.com>,
	"Rusty Russell" <rusty@...tcorp.com.au>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	kvm <kvm@...r.kernel.org>, "Michael Tokarev" <mjt@....msk.ru>,
	"Jesse Barnes" <jbarnes@...tuousgeek.org>
Subject: Re: [PATCH] virtio: make PCI devices take a virtio_pci module ref

On Wed, Dec 10, 2008 at 10:49, Mark McLoughlin <markmc@...hat.com> wrote:
> On Tue, 2008-12-09 at 19:16 +0100, Kay Sievers wrote:
>> On Tue, Dec 9, 2008 at 17:41, Mark McLoughlin <markmc@...hat.com> wrote:
>> > On Mon, 2008-12-08 at 08:46 -0600, Anthony Liguori wrote:
>> >> Mark McLoughlin wrote:
>> >> > On Sun, 2008-12-07 at 18:52 +1030, Rusty Russell wrote:
>> >> >> On Saturday 06 December 2008 01:37:06 Mark McLoughlin wrote:
>> >> >>
>> >> >>> Another example of a lack of an explicit dependency causing problems is
>> >> >>> Fedora's mkinitrd having this hack:
>> >> >>>
>> >> >>>     if echo $PWD | grep -q /virtio-pci/ ; then
>> >> >>>         findmodule virtio_pci
>> >> >>>     fi
>> >> >>>
>> >> >>> which basically says "if this is a virtio device, don't forget to
>> >> >>> include virtio_pci in the initrd too!". Now, mkinitrd is full of hacks,
>> >> >>> but this is a particularly unusual one.
>> >> >>>
>> >> >> Um, I don't know what this does, sorry.
>> >> >>
>> >> >> I have no idea how Fedora chooses what to put in an initrd; I can't think
>> >> >> of a sensible way of deciding what goes in and what doesn't other than
>> >> >> lists and heuristics.
>> >> >>
>> >> >
>> >> > Fedora's mkinitrd creates an initrd suitable to boot the machine you run
>> >> > mkinitrd on, rather than creating an initrd suitable to boot any
>> >> > machine.
>> >> >
>> >> > So, it goes "ah, / is mounted from /dev/vda, we need to include
>> >> > virtio_blk and it's dependencies". It does that in a generic way that
>> >> > works well for most setups:
>> >> >
>> >> >   1) Find the device name (e.g. vda) below /sys/block
>> >> >
>> >> >   2) Follow the 'device' link to e.g. /sys/devices/virtio-pci/virtio1
>> >> >
>> >> >   3) Find the module need for this through either 'modalias' or the
>> >> >      'driver/module' symlink
>> >> >
>> >> >   4) Use modprobe to list any dependencies of that module
>> >> >
>> >> > Clearly, virtio-pci won't be pulled in by any of this so we've added a
>> >> > hack to say "oh, it's a virtio device, let's include virtio_pci just in
>> >> > case".
>> >> >
>> >> > It's not even the case that mkinitrd needs to know how to include the
>> >> > the module for the bus, because in our case that's virtio.ko ... we've
>> >> > pretty effectively hidden the the bus *implementation* from userspace.
>> >> >
>> >> > I don't think this is worth wasting too much time fixing, that's why I'm
>> >> > thinking we should just make virtio_pci built-in by default with
>> >> > CONFIG_KVM_GUEST.
>> >> >
>> >>
>> >> What if we have multiple virtio transports?
>> >
>> > I don't think that's so much an an issue (just build in any transport
>> > supported by KVM), but rather that you might build a non-pv_ops kernel
>> > to run on QEMU which would benefit from using virtio drivers ...
>> >
>> >>   Is there a way that we can
>> >> expose the relationship with virtio-blk and virtio-pci in sysfs?  We
>> >> have a struct device for the PCI device, it's just a matter of making
>> >> the link visible.
>> >
>> > It feels a bit like busy work to generalise this since only virtio_pci
>> > can be built as a module, but here's a patch.
>> >
>> > The mkinitrd hack turns into:
>> >
>> >        # Handle finding virtio bus implementations
>> >        if [ -L ./virtio_module ] ; then
>> >            findmodule $(basename $(readlink ./virtio_module))
>> >        else if echo $PWD | grep -q /virtio-pci/ ; then
>> >            findmodule virtio_pci
>> >        fi; fi
>> >
>> > [PATCH] virtio: add a 'virtio_module' sysfs symlink
>>
>> Doesn't the device have a "driver" link already? If yes, the driver it
>> points to should have a "module" link.
>
> The virtio bus is an abstraction that has several different backend
> implementations - currently virtio-pci, lguest and kvm-s390.
>
> So yes, the driver/module link gives us the device driver, but the
> virtio_module link is to the virtio bus driver (aka implementation,
> transport, backend, ...):
>
>  $> basename $(readlink virtio_module)
>  virtio_pci
>  $> basename $(readlink driver/module)
>  virtio_net

I see. But why not just call it "module", like we do in all other
places, when it points to /sys/module/.

To find dependent modules, you would walk up the chain of parents, and
include everything that is found by looking for "driver/module" and
"module" links?

Wouldn't that make it completely generic, without any virtio specific hacks?

Kay
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