Re: [Qemu-devel] [PATCH 0/6] PCI hotplug improvements

[Alex Williamson]

On Wed, 2012-03-07 at 14:43 +0200, Gleb Natapov wrote:
> On Tue, Mar 06, 2012 at 05:13:36PM -0700, Alex Williamson wrote:
> > Here's a re-work of the patch that added _STA for the purpose of
> > using it as an ack from the guest.  Instead of that, add a notifier
> > for device access.  Once the guest reads from device config space,
> > it owns it.  Until that point, we can remove it directly.  As pointed
> > out by MST, this passes test b) below, which the _STA method would not.
> > As a bonus, no bios change is required for this.  Patches 5 & 6 are
> > just cleanups that can be applied independently.  Thanks,
> > 
> While I agree with Michael that using _STA as ack is a hack I think
> this approach is not less of a hack. It is unlikely that this is how it
> work on bare metal and we should follow real HW if possible.

The test below is the only thing that proved to me it was less of a
hack.  Introducing a _LCK method for a slot may be another way to do
this.  Unfortunately it's not required that the OSPM call _LCK and it's
not mentioned in the msft document referenced previously.
 
> > Tested using Linux guest:
> > a) without acpiphp loaded:
> >     - device_add (nothing happens)
> >     - device_del (device removed directly)
> How it works on real HW? On non ACPI compliant guest hot plug unplug is
> not suppose to work.

We're dealing with ACPI hotplug, so it's not as completely defined as
something like SHPC.  My understanding is that add and remove can be
initiated by either an OS defined software method or via a physical
button on the slot.  What we do in qemu is more akin to the physical
button.  The user places a card in a slot, presses the button, which
will signal the OS to power up the slot, discover the device, and start
making use of it.  An indicator LED is optional in the PCI hotplug spec,
so the user is left to check the OS or look for device activity to
determine if the card was inserted.  If nothing happens, I suspect the
user would likely pull the card back out, which is effectively what
we're allowing here.

> > b) without acpiphp loaded:
> >     - device_add (nothing happens)
> >     - echo 1 > /sys/bus/pci/rescan (device discovered)
> >     - device_del (nothing happens, guest owns device)
> So guest can block a device from being ever removed?

Yes, surprise removal is beyond the scope of this series.  We can always
shutdown a guest to remove the device, but surprise removal risks the
integrity of the guest.  Thanks,

Alex

> >     - modprobe acpiphp
> >     - device_del (guest releases device)
> > c) with acpiphp loaded:
> >     - device_add/del behave as expected (automatic add + coordinated 
> > removal)
> > Tested using WinXP guest:
> >     - device_add/del behave as expected (automatic add + coordinated 
> > removal)
> > 
> > ---
> > 
> > Alex Williamson (6):
> >       api_piix4: Remove PCI_RMV_BASE write code
> >       acpi_piix4: Use pci_get/set_byte
> >       acpi_piix4: Track PCI hotplug status and allow non-ACPI remove path
> >       pci: Add notifier for device probing
> >       acpi_piix4: Only allow writes to PCI hotplug eject register
> >       acpi_piix4: Disallow write to up/down PCI hotplug registers
> > 
> > 
> >  hw/acpi_piix4.c |  175 
> > ++++++++++++++++++++++++++++---------------------------
> >  hw/pci_host.c   |   19 ++++++
> >  hw/pci_host.h   |    2 +
> >  3 files changed, 111 insertions(+), 85 deletions(-)
> 
> --
>                       Gleb.