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Re: [Qemu-trivial] [Qemu-devel] [PATCH] Remove PCI class code from virti
From: |
David Gibson |
Subject: |
Re: [Qemu-trivial] [Qemu-devel] [PATCH] Remove PCI class code from virtio balloon device |
Date: |
Tue, 20 Mar 2012 21:19:47 +1100 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
On Tue, Mar 20, 2012 at 09:54:20AM +0000, Stefan Hajnoczi wrote:
> On Tue, Mar 20, 2012 at 12:42 AM, David Gibson
> <address@hidden> wrote:
> > On Mon, Mar 19, 2012 at 11:33:10AM +0000, Stefan Hajnoczi wrote:
> >> On Mon, Mar 19, 2012 at 03:59:23PM +1100, David Gibson wrote:
> >> > Currently the virtio balloon device, when using the virtio-pci interface
> >> > advertises itself with PCI class code MEMORY_RAM. This is wrong; the
> >> > balloon is vaguely related to memory, but is nothing like a PCI memory
> >> > device in the meaning of the class code, and this code is not required or
> >> > suggested by the virtio PCI specification.
> >> >
> >> > Worse, this patch causes problems on the pseries machine, because the
> >> > firmware, seeing this class code, advertises the device as memory in the
> >> > device tree, and then a guest kernel bug causes it to see this "memory"
> >> > before the real system memory, leading to a crash in early boot.
> >> >
> >> > This patch fixes the problem by removing the bogus PCI class code on the
> >> > balloon device.
> >> >
> >> > Cc: Michael S. Tsirkin <address@hidden>
> >> > Cc: Rusty Russell <address@hidden>
> >> >
> >> > Signed-off-by: David Gibson <address@hidden>
> >> > ---
> >> > hw/virtio-pci.c | 2 +-
> >> > 1 files changed, 1 insertions(+), 1 deletions(-)
> >>
> >> Since this is a guest-visible change we might need to be careful about
> >> how it's introduced.
> >>
> >> Do we need to keep the old class code for existing machine types? The
> >> new class code could be introduced only for 1.1 and later machine types
> >> if we want to be extra careful about introducing guest-visible
> >> changes.
> >
> > So as a general rule, I like to be very careful about user-visible
> > changes. But in this case, I don't think we want to be too hesitant.
> > In particular, it's not just a question of the machine type, but also
> > of how the guest OS will deal with the PCI class code.
> >
> > The class code we were using was Just Plain Wrong. It was not
> > suggetsed by the virtio spec, and it makes no sense. It happens that
> > so far this caused problems only for a guest on a particular machine
> > type, but there's no reason it couldn't cause (different) problems for
> > guests on any machine type.
> >
> > More to the point, it seems reasonably unlikely for existing guests to
> > rely on the broken behaviour: again, there's no reason they'd think
> > they need to based on the spec, and the usual way of matching drivers
> > to PCI devices is with the vendor/device IDs which are correct and not
> > changed by this patch.
> >
> > So, unless we have a known example of an existing guest that would be
> > broken by this change, I think we should implement it ASAP for all
> > machine types.
>
> I agree that in practice the risk is low because working guests are
> probably not using the class code. On the other hand I don't see a
> downside to making this part of the 1.1 machine type,
Well.. there's the fact that I can't what mechanism we would use to
make this per-machine...
> which is what
> users will run when they get this code change anyway. That way we can
> tell users that we never change the device model in a release with a
> straight face :).
>
> Anthony: I'm not sure how strict we are about a user-visible change like this?
>
> Stefan
>
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
- [Qemu-trivial] [PATCH] Remove PCI class code from virtio balloon device, David Gibson, 2012/03/15
- [Qemu-trivial] [PATCH] Remove PCI class code from virtio balloon device, David Gibson, 2012/03/19
- Re: [Qemu-trivial] [Qemu-devel] [PATCH] Remove PCI class code from virtio balloon device, Stefan Hajnoczi, 2012/03/19
- Re: [Qemu-trivial] [Qemu-devel] [PATCH] Remove PCI class code from virtio balloon device, David Gibson, 2012/03/19
- Re: [Qemu-trivial] [Qemu-devel] [PATCH] Remove PCI class code from virtio balloon device, Stefan Hajnoczi, 2012/03/20
- Re: [Qemu-trivial] [Qemu-devel] [PATCH] Remove PCI class code from virtio balloon device,
David Gibson <=
- Re: [Qemu-trivial] [Qemu-devel] [PATCH] Remove PCI class code from virtio balloon device, Stefan Hajnoczi, 2012/03/21
- Re: [Qemu-trivial] [Qemu-devel] [PATCH] Remove PCI class code from virtio balloon device, Stefan Hajnoczi, 2012/03/21
- Re: [Qemu-trivial] [Qemu-devel] [PATCH] Remove PCI class code from virtio balloon device, David Gibson, 2012/03/21
- Re: [Qemu-trivial] [Qemu-devel] [PATCH] Remove PCI class code from virtio balloon device, Michael S. Tsirkin, 2012/03/21
- Re: [Qemu-trivial] [Qemu-devel] [PATCH] Remove PCI class code from virtio balloon device, Anthony Liguori, 2012/03/21
- Re: [Qemu-trivial] [Qemu-devel] [PATCH] Remove PCI class code from virtio balloon device, Michael S. Tsirkin, 2012/03/21
- Re: [Qemu-trivial] [Qemu-devel] [PATCH] Remove PCI class code from virtio balloon device, Anthony Liguori, 2012/03/21
- Re: [Qemu-trivial] [Qemu-devel] [PATCH] Remove PCI class code from virtio balloon device, Michael S. Tsirkin, 2012/03/21
- Re: [Qemu-trivial] [Qemu-devel] [PATCH] Remove PCI class code from virtio balloon device, Anthony Liguori, 2012/03/21
- Re: [Qemu-trivial] [Qemu-devel] [PATCH] Remove PCI class code from virtio balloon device, Anthony Liguori, 2012/03/21