Re: [Xen-devel] PCIe devices that are hotplugged after MMIO has been setup fail due to _CRS not covering 64-bit area

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Wed, Sep 28, 2016 at 03:21:08AM -0600, Jan Beulich wrote:
> >>> On 27.09.16 at 16:43, <konrad.wilk@xxxxxxxxxx> wrote:
> > If the guest is booted with 'pci' we nicely expand the MMIO region below
> > 4GB and try to fit in the BARs in there. If that fails (not enough
> > space) we move it above the memory (64-bit). And throughout all of this
> > we also update the _CRS field to cover these ranges.
> > 
> > (Note, I need to check if the 64-bit area is also set, I think it is).
> > 
> > But the situation is different if we hot-plug a device that has too big
> > BAR to fit in the MMIO region. We move it in the 64-bit area but we
> > don't update the _CRS. Which means that Linux will complain (unless
> > booted with pci=nocrs)). Not sure about Windows but I would assume so
> > to.
> > 
> > I was wondering what would be a good way to solve this? I looked at some
> > Dell machines to see how they deal with hotplug PCIe devices and they
> > just declared all the memory in the _CRS (including RAM).
> > 
> > We could do a hybrid - during bootup make the _CRS region have entry from
> > end of RAM to .. end of memory?
> 
> End of physical address space you mean? Generally yes, but we

Yes.
> need to be a little careful there: For one, on AMD we'd better not
> overlap with the HT area. And then there's this MTRR related
> comment next to the setting of pci_hi_mem_end (albeit both HT
> area start and end of PA space should be aligned well enough).

<nods>
> 
> > Or perhaps add some extra logic between QEMU and ACPI AML to expand (or
> > perhaps modify the last _CRS entry) when PCIe devices are hotplugged?
> 
> While that would be the most flexible variant, I'd be afraid of this
> getting rather complicated. Or have you already got some
> reasonable layout of how this would look like?

Nothing yet sadly, just soliciting input at this point.

Thanks again for the tidbit about HT.
> 
> Jan
> 

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