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

[Gleb Natapov]

On Mon, Mar 05, 2012 at 08:26:23AM +0200, Michael S. Tsirkin wrote:
> 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
> 
Why have this up, down things at all? What's wrong with how CPU hotplug
works. It has only one HW register that returns a single bitmask that
has 1 for available cpu and 0 for non available. AML can figure out what
changed by having local copy of the old register's value to compare new value
with.

--
                        Gleb.