[CHERRY-PICK] MdeModulePkg: PciBusDxe: Degrade MEM64 to PMEM64 when bridge lacks MEM64#1690
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kuqin12 merged 1 commit intomicrosoft:release/202511from Mar 10, 2026
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…ridge lacks MEM64 P2P bridge Memory Base/Limit registers (0x20-0x22) only support 32-bit addresses. Previously, MEM64 resources behind such bridges were always degraded to MEM32, forcing allocation below 4GB. This can lead to resource constraints when using massive storage (such as MPF drives) as MEM32 space is limited on most platforms. Based on PCIe Base Specification 6.3+, assigning 64-bit resources irrespective of the prefetchable/non-prefetchable BAR bit is allowed. In DegradeResource(), if an upstream bridge supports PMEM64, MEM64 resources are now degraded to PMEM64 first. This enables 64-bit BAR allocation for devices like NVMe controllers that declare non- prefetchable 64-bit BARs. We still fall back to MEM32 degradation if the bridge lacks PMEM64 support as well. Ref: PCI-SIG ECN "Removing Prefetchable Terminology" (2024-04-05) Ref: PCIe Base Specification 6.3+ Signed-off-by: Kun Qin <kuqin12@gmail.com> (cherry picked from commit f6489621b8ae1164c5e4930902988ba7c86847ba)
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Description
P2P bridge Memory Base/Limit registers (0x20-0x22) only support 32-bit addresses. Previously, MEM64 resources behind such bridges were always degraded to MEM32, forcing allocation below 4GB. This can lead to resource constraints when using massive storage (such as MPF drives) as MEM32 space is limited on most platforms.
Based on PCIe Base Specification 6.3+, assigning 64-bit resources irrespective of the prefetchable/non-prefetchable BAR bit is allowed.
In DegradeResource(), if an upstream bridge supports PMEM64, MEM64 resources are now degraded to PMEM64 first. This enables 64-bit BAR allocation for devices like NVMe controllers that declare non- prefetchable 64-bit BARs.
We still fall back to MEM32 degradation if the bridge lacks PMEM64 support as well.
Ref: PCI-SIG ECN "Removing Prefetchable Terminology" (2024-04-05)
Ref: PCIe Base Specification 6.3+
How This Was Tested
This was tested on propreitary physical platform and verified that the PCIe devices behind P2P bridge can properly request 64bit resources without downgrading.
Integration Instructions
N/A