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[ARTICLE · art-142695] src=forum.level1techs.com ↗ pub= topic=ai-infrastructure verified=true sentiment=· neutral

AM4 motherboard with quad R9700's behind plx88096 ROMBAR Woes

A user on an AM4 X570-D4U-2L2T/BCM motherboard resolved missing PCIe BAR space that blocked four R9700 AI GPUs behind a PLX88096 switch by disabling Legacy CSM support in the BIOS, which also restored the onboard 10GbE controller. The fix left peer-to-peer (P2P) disabled in AMD-SMI topology inside the VM passed through all four devices, with the access table showing only self-links ENABLED between 0000:01:00.0 through 0000:04:00.0. The user had applied kernel arguments including pci=realloc=on, amd_iommu=on, iommu=pt and pcie_aspm=off before finding the CSM setting was the cause.

read1 min views2 publishedSep 30, 2026
AM4 motherboard with quad R9700's behind plx88096 ROMBAR Woes
Image: Forum (auto-discovered)

So i recently got my PLX88096 chip in the mail, and have used it to install 4 R9700 AI’s onto the system, however it seems my poor prosumer grade AM4 X570-D4U-2L2T/BCM motherboard has had enough of my shenanigans and just doesnt have the BAR space to deal with it

My onboard 10Gbe also stopped working, but i suspect this is just a side effect of the lack of BAR space

I’ve added the Dmesg logs and my kernel opts

Please obi-wan janitobi’s, you guys are my only hope:

Kernel Args:

GRUB_CMDLINE_LINUX_DEFAULT=“quiet amd_iommu=on iommu=pt pcie_aspm=off amdgpu.ras_enable=0 pci=realloc=on amdgpu.runpm=0 amdgpu.gpu_recovery=1 pcie_acs_override=downstream,multifunction, default_hugepagesz=1G hugepagesz=1G hugepages=108 transparent_hugepage=always”

DMESG: See attachment

[dmesg.txt](https://forum.level1techs.com/uploads/short-url/cuEyL12crKwewszQIO6ydsM2IAr.txt) (46.6 KB)

For your viewing pleasure(or distaste, depending on tolerance):

It WORKS!!!, it was Legacy CSM support being enabled, i have a new problem now though, even though everything is now properly showing inside the VM P2P is still disabled in AMD-SMI topology:

ACCESS TABLE:

0000:01:00.0 0000:02:00.0 0000:03:00.0 0000:04:00.0 0000:01:00.0 ENABLED DISABLED DISABLED DISABLED

0000:02:00.0 DISABLED ENABLED DISABLED DISABLED

0000:03:00.0 DISABLED DISABLED ENABLED DISABLED

0000:04:00.0 DISABLED DISABLED DISABLED ENABLED

WEIGHT TABLE:

this is inside the VM that’s being passed through all these devices

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