Re: [PATCH][kvmtool] virtio/pci: Correctly handle MSI-X masking while MSI-X is disabled

[Marc Zyngier <maz@kernel.org>]

On Tue, 24 Aug 2021 15:32:53 +0100,
Marc Zyngier <maz@kernel.org> wrote:
> 
> Hi Andre,
> 
> On Mon, 23 Aug 2021 17:48:33 +0100,
> Andre Przywara <andre.przywara@arm.com> wrote:
> > 
> > On Sat, 21 Aug 2021 13:07:42 +0100
> > Marc Zyngier <maz@kernel.org> wrote:
> > 
> > Hi Marc,
> > 
> > > Since Linux commit 7d5ec3d36123 ("PCI/MSI: Mask all unused MSI-X
> > > entries"), kvmtool segfaults when the guest boots and tries to
> > > disable all the MSI-X entries of a virtio device while MSI-X itself
> > > is disabled.
> > > 
> > > What Linux does is seems perfectly correct. However, kvmtool uses
> > > a different decoding depending on whether MSI-X is enabled for
> > > this device or not. Which seems pretty wrong.
> > 
> > While I really wish this would be wrong, I think this is
> > indeed how this is supposed to work: The Virtio legacy spec makes the
> > existence of those two virtio config fields dependent on the
> > (dynamic!) enablement status of MSI-X. This is reflected in:
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/uapi/linux/virtio_pci.h#n72
> > and explicitly mentioned as a footnote in the virtio 0.9.5 spec[1]:
> > "3) ie. once you enable MSI-X on the device, the other fields move. If
> > you turn it off again, they move back!"
> 
> Madness! What was Rusty on at the time? I really hope the bitcoin
> thing is buying him better stuff...
> 
> > I agree that this looks like a bad idea, but I am afraid we are stuck
> > with this. It looks like the Linux driver is at fault here, it should
> > not issue the config access when MSIs are disabled. Something like this
> > (untested):
> > 
> > --- a/drivers/virtio/virtio_pci_legacy.c
> > +++ b/drivers/virtio/virtio_pci_legacy.c
> > @@ -103,6 +103,9 @@ static void vp_reset(struct virtio_device *vdev)
> >  
> >  static u16 vp_config_vector(struct virtio_pci_device *vp_dev, u16 vector)
> >  {
> > +       if (!vp_dev->msix_enabled)
> > +               return VIRTIO_MSI_NO_VECTOR;
> > +
> >         /* Setup the vector used for configuration events */
> >         iowrite16(vector, vp_dev->ioaddr + VIRTIO_MSI_CONFIG_VECTOR);
> >         /* Verify we had enough resources to assign the vector */
> > 
> > This is just my first idea after looking at this, happy to stand
> > corrected or hear about a better solution.
> 
> I don't think this works. It instead completely disables MSI-X, which
> is a total bore. I think the only way to deal with it is to quirk it
> to prevent the bulk masking to take effect before MSI-X is enabled.

Actually, let me correct myself. I tested the wrong configuration (why
isn't --force-pci the bloody default in kvmtool?). This patch doesn't
fix anything at all, and kvmtool just explodes.

Having dug further, it isn't the config space that causes problems,
but the programming of the MSI-X vectors. I'm starting to suspect the
layout of the MSI-X bar in kvmtool.

	M.

-- 
Without deviation from the norm, progress is not possible.