Re: [RFC] xen/x86: allow overlaps with non-RAM regions

[Jan Beulich <jbeulich@xxxxxxxx>]

On 10.04.2025 22:55, Jason Andryuk wrote:
> On 2025-04-04 06:28, Roger Pau Monné wrote:
>> On Thu, Apr 03, 2025 at 06:01:42PM -0700, Stefano Stabellini wrote:
>>> On one Sapphire AMD x86 board, I see this:
>>>
>>>
>>> (XEN) [0000003943ca6ff2]  [00000000f0000000, 00000000f7ffffff] (reserved)
>>> (XEN) [00000039460886d9]  [00000000fd000000, 00000000ffffffff] (reserved)
>>> [...]
>>> (XEN) [    4.612235] 0000:02:00.0: not mapping BAR [fea00, fea03] invalid 
>>> position
>>>
>>>
>>> Linux boots fine on this platform but Linux as Dom0 on Xen does not.
>>> This is because the pci_check_bar->is_memory_hole check fails due to the
>>> MMIO region overlapping with the EFI reserved region.
>>
>> That's weird.  (Partially) the reason to not attempt to map such BAR
>> is that it should already be mapped, because at dom0 creation time all
>> reserved regions are added to the p2m (see arch_iommu_hwdom_init()).
>> If that's not the case we should figure out why this reserved region
>> is not added to dom0 p2m as part of arch_iommu_hwdom_init().
> 
> Victor discovered these regions are type 11 EfiMemoryMappedIO, but they 
> get converted to e820 RESERVED.  The BAR points into it.
> 
> 00000f0000000-00000f7ffffff type=11 attr=800000000000100d
> 00000fd000000-00000fedfffff type=11 attr=800000000000100d
> 00000fee00000-00000fee00fff type=11 attr=8000000000000001
> 00000fee01000-00000ffffffff type=11 attr=800000000000100d
> 
> Xenia discovered Linux keeps small regions like this reserved, but lets 
> larger ones (>= 256kb) become holes.  See the comment in Linux 
> arch/x86/platform/efi/efi.c:efi_remove_e820_mmio() around line 301.

What a hack. And the mentioning of MMCFG space isn't even correct there,
I think. That space may legitimately be reserved (and at least older
Linux actually checked for that), unlike any ranges where BARs may live.
As to host bridge windows - I dare to question that they always need to
be "large". Similarly nothing guarantees the some non-window space
included in _CRS may not point at a "large" region.

Jan