Re: [Xen-devel] [RFC] Disable xen platform device when it is not needed.

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 19/02/2009 09:39, "Keir Fraser" <keir.fraser@xxxxxxxxxxxxx> wrote:

>> I will create the patch to add "xen_platform_pci" option in guest
>> config file to disable xen platform device.
>> 
>>     xen_platform_pci=0: Disable xen platform device.
>>     xen_platform_pci=1: Enable xen platform device. (default)
> 
> I still might veto this. I don't really want the platform device I/O ports
> scattered elsewhere for this rather frivolous reason.

Actually it's okay, you can alias the 'platform_flags' byte of the ioport
BAR to fixed byte port 0x10. There's a handler for that already in
xen_platform.c (platform_fixed_ioport_{read,write}1). That's the only port
that hvmloader/rombios cares about.

Hvmloader/rombios can then always use fixed single-byte ioport 0x10. This is
actually better since the code is simpler and also rombios accesses that
port on S3 suspend/resume, at which point an OS could have relocated the
xen_platform ioport BAR! Using a fixed port is therefore safer.

Notice that I said you must *alias* this ioport byte. You cannot remove it
from the xen_platform BAR as old saved guests will rely on it being there,
and removing it will break their S3. We have to be backward compatible with
the Xen-3.3 virtual hardware interface for saved guests.

So, no veto after all!

 -- Keir



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