linux-kernel - Re: [PATCH] virtio: make PCI devices take a virtio

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Message-ID: <ac3eb2510812091016u944d6c9je6ac8470127e5115@mail.gmail.com>
Date:	Tue, 9 Dec 2008 19:16:05 +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 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
>
> Cheers,
> Mark.
>
> [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.

Thanks,
Kay
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