Re: [Qemu-devel] [PATCH] seabios: acpi: Add _STA for PCI hotplug slots

["Michael S. Tsirkin" <mst@redhat.com>]

On Sun, Mar 04, 2012 at 08:30:00PM -0700, Alex Williamson wrote:
> On Sun, 2012-03-04 at 20:53 +0200, Michael S. Tsirkin wrote:
> > On Fri, Feb 24, 2012 at 04:21:17PM -0700, Alex Williamson wrote:
> > > When a Status method is provided on a slot, the OSPM evaluates
> > > _STA in response to the device check notify on the slot.  This
> > > allows some degree of a handshake between the platform and the
> > > OSPM that the hotplug has been acknowledged.
> > >
> > > In order to implement _STA, we need to know which slots have
> > > devices.  A slot with device returns 0x0F, a slot without a
> > > device returns Zero.  We get this information from Qemu using
> > > the 0xae08 I/O port register.  This was previously the read-side
> > > of the register written to commit a device eject and always
> > > returned 0 on read.  It now returns a bitmap of present slots,
> > > so we know that reading 0 means we have and old Qemu and
> > > dynamically modify our SSDT to rename the _STA methods.  This
> > > is necessary to allow backwards compatibility.

...

> > > The _STA method also writes the slot identifier to I/O port
> > > register 0xae00 as an acknowledgment of the hotplug request.


To summarize my previous messages, my notes are
- not clear that we want to implement _STA: yes we can tell hypervisor
  what did _STA report to OSPM but this won't be needed without _STA
- assuming we do, it seems clear that we want hypervisor
  to know what it is that we told OSPM about slot status
- the specific interface used for the above is fairly tricky
  so it needs documentation explaining how both sides cooperate

-- 
MST