[xen master] x86/mcfg: relax memory map checks on newer firmware

[patchbot@xxxxxxx]

commit 87279cafc0b6ba7078576d7f8355caee0cc9598a
Author:     Roger Pau Monne <roger.pau@xxxxxxxxxx>
AuthorDate: Wed Jun 3 14:58:33 2026 +0200
Commit:     Roger Pau Monne <roger.pau@xxxxxxxxxx>
CommitDate: Fri Jun 5 17:40:21 2026 +0200

    x86/mcfg: relax memory map checks on newer firmware
    
    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.
    
    Recent Lenovo systems have been found to have the MMCFG region in an
    E820 hole, which looks to be technically spec compliant, but is rejected
    by Xen.
    
    The 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.
    
    Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
---
 xen/arch/x86/x86_64/mmconfig-shared.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/x86_64/mmconfig-shared.c 
b/xen/arch/x86/x86_64/mmconfig-shared.c
index d0cbc15170..b33e2f56e6 100644
--- a/xen/arch/x86/x86_64/mmconfig-shared.c
+++ b/xen/arch/x86/x86_64/mmconfig-shared.c
@@ -13,6 +13,7 @@
  */
 
 #include <xen/acpi.h>
+#include <xen/dmi.h>
 #include <xen/init.h>
 #include <xen/mm.h>
 #include <xen/param.h>
@@ -369,12 +370,15 @@ static bool __init pci_mmcfg_reject_broken(void)
     typeof(pci_mmcfg_config[0]) *cfg;
     int i;
     bool valid = true;
+    int year;
 
     if ((pci_mmcfg_config_num == 0) ||
         (pci_mmcfg_config == NULL) ||
         (pci_mmcfg_config[0].address == 0))
         return 0;
 
+    dmi_get_date(DMI_BIOS_DATE, &year, NULL, NULL);
+
     for (i = 0; i < pci_mmcfg_config_num; i++) {
         u64 addr, size;
 
@@ -390,7 +394,13 @@ static bool __init pci_mmcfg_reject_broken(void)
                (unsigned int)cfg->start_bus_number,
                (unsigned int)cfg->end_bus_number);
 
-        if ( !is_mmconf_reserved(addr, size, i, cfg) ||
+        /*
+         * For firmwares prior to 2016, confirm that MMCFG is marked as
+         * reserved.  For 2016 and later, also allow MMCFG being in a hole.
+         */
+        if ( ((year < 2016 || !is_memory_hole(maddr_to_mfn(addr),
+                                              maddr_to_mfn(addr + size - 1))) 
&&
+              !is_mmconf_reserved(addr, size, i, cfg)) ||
              pci_mmcfg_arch_enable(i) < 0 )
         {
             pci_mmcfg_arch_disable(i);
--
generated by git-patchbot for /home/xen/git/xen.git#master