Re: [PATCH for-4.22 v2 2/2] x86/mcfg: relax memory map checks on newer firmware

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

On 6/4/26 3:25 PM, Roger Pau Monne wrote:
> Per PCI Firmware 3.3 specification, section 4.1.2, ECAM space must be
> reserved by declaring a motherboard resource, but there's no requirement to
> mention it in E820.  The specification additionally states that: the
> resources can optionally be returned in Int15 E820h or EFIGetMemoryMap as
> reserved memory.
>
> This fixes an issue with detection of extended capabilities when running
> Xen on a Lenovo system that doesn't list the MCFG area as an
> EfiMemoryMappedIO region (or as any other type) in the EFI memory map.
>
> The more strict logic was introduced in Linux in 2006 as 946f2ee5c731
> ("[PATCH] i386/x86-64: Check that MCFG points to an e820 reserved area").
> This was picked up by Xen when MCFG support was added in 3b35911d709e
> ("Enable pci mmcfg and ATS for x86_64").  Apply an approach similar to
> what Linux has done in 199f968f1484 ("x86/pci: Skip early E820 check for
> ECAM region") and relax the strict reserved region checking so it's only
> done for firmware manufactured prior to 2016.  For firmware from 2016 and
> newer allow MCFG region to reside in holes on the memory map.  Note Xen is
> still more strict than Linux however, as it will refuse to use MCFG regions
> that overlap with memory map regions different than reserved.
>
> When dom0 boots it can prevent access to misconfigured MCFG regions by
> using the PHYSDEVOP_pci_mmcfg_reserved hypercall.  This brings Xen's early
> usage of MCFG (prior to ACPI AML parsing) more in line with the
> implementation in Linux.
Don't we want to add Fixes: or Ammends: here?

> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Generally it looks low-risk patch so:
 Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>

Thanks.

~ Oleksii