Re: [PATCH] PCI: Reserve prefetchable window headroom for Resizable BARs

[Christian König]

On 8/31/26 18:58, Ilpo Järvinen wrote:
> On Mon, 31 Aug 2026, Christian König wrote:
...
>>> 2. Pack Oversized Windows (BAR Demotion): 
>>> The PCI-to-PCI Bridge spec (r1.2, sec 3.2.5) permits a prefetchable BAR 
>>> to be assigned from the non-prefetchable window. By clearing the PREFETCH 
>>> flag on small control BARs during enumeration, they are placed below 4 GiB. 
>>> The prefetchable window is then cleanly sized to the large BAR alone.
>>
>> Interesting hack, shouldn't really matter for AMD GPUs but that is 
>> something more on the heuristic side. 
>>
>> A perfectly valid trick which is done by BIOS implementations but what 
>> the Linux PCI subsystem still hasn't learned are back to back 
>> allocations, e.g. something like this:
>>
>> BAR0 32 GiB of GPU #1
>> BAR1 2	MiB of GPU #1
>>
>> BAR1 2	MiB of GPU #2
>> BAR0 32 GiB of GPU #2
> 
> Assuming you're oversimplifying the layout above, this was added by the commit 
> 9036bd0efcb6 ("PCI: Align head space better").

Oh nice! Yeah we had quite a number of bug reports because of that.

Usually hot unplugging some PCIe HW and then re-plugging it caused surprises when the BIOS assigned the resources initially and then the Linux kernel on re-plug.

I need to dig up those bug reports and add a reference to the commit to let people retest. 

Thanks for that pointer,
Christian.