Re: [Qemu-devel] [PATCH RFC 0/6] q35: add acpi pci hotplug support

[Michael S. Tsirkin]

On Mon, Jul 03, 2017 at 09:26:33PM +0300, Marcel Apfelbaum wrote:
> On 03/07/2017 19:34, Michael S. Tsirkin wrote:
> > On Mon, Jul 03, 2017 at 02:27:11PM +0200, Igor Mammedov wrote:
> > > On Fri, 30 Jun 2017 10:25:05 +0300
> > > Marcel Apfelbaum <address@hidden> wrote:
> > > 
> > > [...]
> > > > 
> > > > So for the modern systems not supporting PCI ACPI hotplug
> > > > we don't need pci-bridges anyway, but for the older ones
> > > > the ACPI code of the pci-bridge will be loaded into the
> > > > ACPI namespace only if a pci-bridge is actually hot-plugged.
> > > 
> > > just note that the set of 'older' guest OSes is limited to
> > > one that do not support SHPC (i.e. to EOLed WinXP & co)
> > > as for linux and more modern Windows SHPC hotplug should
> > > just work without our ACPI hack (which taxes low memory
> > > to keep acpi tables for bridges).
> > > 
> > > So I'm in favor of Michael's suggestion to leave ACPI PCI
> > > only in PC machine for old WinXP guests and to keep Q35
> > > clean, where linux or newer Windows guests could just
> > > use standard SHPC.
> > > 
> > > [...]
> > 
> > I didn't realize windows actually supports SHPC for PCI.
> 
> Me neither, if Igor is right I am all for shpc hotplug
> since Q35 is not supposed to support older guests.
> 
> I remember I succeeded to enable shpc hotplug some time
> ago, but only for Linux guests.
> 
> Igor, do you have some spec/doc on newer Windows OSes that confirm
> PCI shpc hotplug support?

Just try it, easier than poking at specs which aren't always up to date.

> > 
> > Do they correctly set _OSC Arg3, bit offset 1?
> >     SHPC Native Hot Plug control
> >     The OS sets this bit to 1 to request control over PCI/PCI-X Standard 
> > Hot-Plug Controller
> >     (SHPC) hot plug. If the OS successfully receives control of this 
> > feature, it must track and
> >     update the status of hot plug slots and handle hot plug events as 
> > described in the SHPC
> >     Specification.
> > I was under impression they only set bit 0.
> > 
> 
> Alexandr, if modern Windows OSes do support shpc, it makes our
> job easier, can you please try to enable shpc hotplug?
> 
> Thanks,
> Marcel

No need to enable or even have a bridge for that at all -
set the bit in _OSC, see what does guest enable.