Re: [Qemu-devel] [PATCH V2 2/4] pci: reserve 64 bit MMIO range for PCI hotplug

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

On Wed, May 18, 2016 at 05:52:29PM +0300, Marcel Apfelbaum wrote:
> On 05/18/2016 05:42 PM, Michael S. Tsirkin wrote:
> >On Wed, May 18, 2016 at 04:31:46PM +0200, Igor Mammedov wrote:
> >>On Wed, 18 May 2016 17:12:09 +0300
> >>Marcel Apfelbaum <marcel@redhat.com> wrote:
> >>
> >>>On 05/18/2016 05:11 PM, Michael S. Tsirkin wrote:
> >>>>On Wed, May 18, 2016 at 03:59:29PM +0200, Igor Mammedov wrote:
> >>>>>On Sun, 15 May 2016 22:23:32 +0300
> >>>>>Marcel Apfelbaum <marcel@redhat.com> wrote:
> >>>>>
> >>>>>>Using the firmware assigned MMIO ranges for 64-bit PCI window
> >>>>>>leads to zero space for hot-plugging PCI devices over 4G.
> >>>>>>
> >>>>>>PC machines can use the whole CPU addressable range after
> >>>>>>the space reserved for memory-hotplug.
> >>>>>>
> >>>>>>Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
> >>>>>that patch also has side effect of unconditionally adding
> >>>>>QWordMemory() resource in PCI0._CRS
> >>>>>on all machine types with QEMU generated ACPI tables.
> >>>>>
> >>>>>Have you tested that it won't break boot of legacy OSes
> >>>>>(XP, WS2003, old linux with 32bit kernel)?
> >>>>
> >>>>It's almost sure it break it.
> >>>>Maybe you can check _REV in _CRS to work around this for XP.
> >>>
> >>>I'll try it.
> >>but only after you check if just presence of QWord would crash XP,
> >>so in case it doesn't crash we would keep _CRS simple static
> >>structure.
> >>
> >>I very vaguely recall that XP ignored QWord in PCI0._CRS,
> >>but it was long time ago so it won't hurt to recheck.
> >
> >I played with different guests (32 and 64 bit)
> >at some point.
> >
> >Generally, windows tends to crash when CRS resources exceed the
> >supported limits
> >of physical memory (sometimes with weird off by one errors,
> >e.g. win7 32 bit seems to survive with a 36 bit pci hole even though
> >this means the max address is 2^37-1 which it can't address).
> >
> >This might depend on CPU as well.
> 
> It seems QEMU returns 40-bit as CPU addressable bits, but I might got it wrong.
> 
> >
> >Which makes me ask: why don't we fix this in BIOS?
> >If you want it to allocate a large window, do it.
> 
> I don't follow, BIOS can assign resources to PCI devices, but how to
> specify a MMIO range for hot-plug? Add it to the CRS of the host-bridge, right?
> 
> Thanks,
> Marcel
> 
> [...]

Ah I forgot on PC there's no register for this. On Q35 there is.

-- 
MST