PCIe Pass-through lanes and lane performance

[JamesG]

@anglerfish27 Supermicro H12SSL-i

I've dabbled with some PCIe settings, but I'm not really getting anywhere.

Here's a link to the manual:

https://www.supermicro.com/manuals/motherboard/EPYC7000/MNL-2314.pdf

Thanks!

[john.c]

@JamesG said:

@anglerfish27 Supermicro H12SSL-i

I've dabbled with some PCIe settings, but I'm not really getting anywhere.

Here's a link to the manual:

https://www.supermicro.com/manuals/motherboard/EPYC7000/MNL-2314.pdf

Thanks!

Enable the SVM option will initialise the virtualisation settings automatically. As well as the SR-IOV settings, there’s going to other things need doing in XOA, to configure the necessary settings so that GPU can be passed through to the VM.

[TeddyAstie]

@JamesG

I know I'm a bit out in uncharted territory here. I don't think XCP-ng supports ReBAR via pass-through (I think that's on the road-map for XenServer9 and maybe XCP-ng 9 as well), current gen GPUs all want ReBAR support, Intel GPU's are in a mostly experimental/developmental mode and might be having a murky future anyway...

XCP-ng guest firmware have a very "legacy" initial BAR placement (i.e thus no explicit support for Resizable BAR, everything below 4G, ...), but the guest operating system can still replace the location of BAR itself if the hardware allows it (in particular Linux will try it, but I'm not sure Windows does).

In your case, the GPU VRAM BAR is 16 GB, so it's obviously resized. And according to xe, you should see in dmesg something like BAR2 resized to 16384MiB

[TeddyAstie]

@JamesG said:

When I ask the system what it thinks the capabilities are, again I get Gen1x1:

cat /sys/bus/pci/devices/0000:00:08.0/current_link_speed
2.5 GT/s PCIe
cat /sys/bus/pci/devices/0000:00:08.0/current_link_width
1
cat /sys/bus/pci/devices/0000:00:08.0/max_link_speed
2.5 GT/s PCIe
cat /sys/bus/pci/devices/0000:00:08.0/max_link_width
1

This information is actually crafted by QEMU, it doesn't represent the actual real device PCI state; you should check this from Dom0 which should be closer to reality.

[JamesG]

@john.c

Yes indeed. SVM has been enabled as well as SR-IOV and IOMMU.

That should be all good for full virtualization and support pass-through and SR-IOV features.

[JamesG]

@TeddyAstie

I haven't cleared the PCI pass-through masking in Dom0, but below is the same lspci command from Dom0. I'm pretty sure I'm running the latest 8.3 with all the current patches. This could just be a kernel issue?

lspci -vvxxxx -s 0000:83:00.0
83:00.0 VGA compatible controller: Intel Corporation Device e212 (prog-if 00 [VGA controller])
	Subsystem: Intel Corporation Device 1114
	Control: I/O- Mem- BusMaster- SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B- DisINTx-
	Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
	Region 0: Memory at 20002000000 (64-bit, prefetchable) [disabled] [size=16M]
	Region 2: Memory at 1f800000000 (64-bit, prefetchable) [disabled] [size=16G]
	Expansion ROM at f7c00000 [disabled] [size=2M]
	Capabilities: [40] Vendor Specific Information: Len=0c <?>
	Capabilities: [70] Express (v2) Endpoint, MSI 00
		DevCap:	MaxPayload 256 bytes, PhantFunc 0, Latency L0s unlimited, L1 unlimited
			ExtTag+ AttnBtn- AttnInd- PwrInd- RBE+ FLReset+ SlotPowerLimit 0.000W
		DevCtl:	Report errors: Correctable+ Non-Fatal+ Fatal+ Unsupported-
			RlxdOrd+ ExtTag+ PhantFunc- AuxPwr- NoSnoop+ FLReset-
			MaxPayload 256 bytes, MaxReadReq 512 bytes
		DevSta:	CorrErr- UncorrErr- FatalErr- UnsuppReq- AuxPwr- TransPend-
		LnkCap:	Port #0, Speed 2.5GT/s, Width x1, ASPM L0s L1, Exit Latency L0s <64ns, L1 <1us
			ClockPM- Surprise- LLActRep- BwNot- ASPMOptComp+
		LnkCtl:	ASPM Disabled; RCB 64 bytes Disabled- CommClk-
			ExtSynch- ClockPM- AutWidDis- BWInt- AutBWInt-
		LnkSta:	Speed 2.5GT/s, Width x1, TrErr- Train- SlotClk- DLActive- BWMgmt- ABWMgmt-
		DevCap2: Completion Timeout: Range B, TimeoutDis+, LTR+, OBFF Not Supported
		DevCtl2: Completion Timeout: 50us to 50ms, TimeoutDis-, LTR-, OBFF Disabled
		LnkCtl2: Target Link Speed: 2.5GT/s, EnterCompliance- SpeedDis-
			 Transmit Margin: Normal Operating Range, EnterModifiedCompliance- ComplianceSOS-
			 Compliance De-emphasis: -6dB
		LnkSta2: Current De-emphasis Level: -6dB, EqualizationComplete-, EqualizationPhase1-
			 EqualizationPhase2-, EqualizationPhase3-, LinkEqualizationRequest-
	Capabilities: [ac] MSI: Enable- Count=1/1 Maskable+ 64bit+
		Address: 0000000000000000  Data: 0000
		Masking: 00000000  Pending: 00000000
	Capabilities: [d0] Power Management version 3
		Flags: PMEClk- DSI- D1- D2- AuxCurrent=0mA PME(D0+,D1-,D2-,D3hot+,D3cold-)
		Status: D0 NoSoftRst+ PME-Enable- DSel=0 DScale=0 PME-
	Capabilities: [100 v1] Alternative Routing-ID Interpretation (ARI)
		ARICap:	MFVC- ACS-, Next Function: 0
		ARICtl:	MFVC- ACS-, Function Group: 0
	Capabilities: [110 v1] #00
	Capabilities: [200 v1] Address Translation Service (ATS)
		ATSCap:	Invalidate Queue Depth: 00
		ATSCtl:	Enable-, Smallest Translation Unit: 00
	Capabilities: [420 v1] #15
	Capabilities: [220 v1] #24
	Capabilities: [320 v1] Single Root I/O Virtualization (SR-IOV)
		IOVCap:	Migration-, Interrupt Message Number: 000
		IOVCtl:	Enable- Migration- Interrupt- MSE- ARIHierarchy+
		IOVSta:	Migration-
		Initial VFs: 2, Total VFs: 2, Number of VFs: 0, Function Dependency Link: 00
		VF offset: 1, stride: 1, Device ID: e212
		Supported Page Size: 00000553, System Page Size: 00000001
		Region 0: Memory at 0000020000000000 (64-bit, prefetchable)
		Region 2: Memory at 000001fc00000000 (64-bit, prefetchable)
		VF Migration: offset: 00000000, BIR: 0
	Capabilities: [400 v1] Latency Tolerance Reporting
		Max snoop latency: 0ns
		Max no snoop latency: 0ns
	Kernel driver in use: pciback
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20: 00 00 00 00 00 00 00 00 00 00 00 00 86 80 14 11
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[JamesG]

Not to be a nuisance...But just curious if there are other thoughts on this.

[TeddyAstie]

@JamesG Can you try having a workload on the GPU (and also checking if the performance is lower, or if it's just display) ?

[MajorP93]

Hello!
Just an idea but maybe the PCI link speed you are seeing is related to some sort of energy saving feature?
Background: I have a Nvidia GPU in my PC and when there is no load the PCIe speed is lowered from PCIe 4.0 to 1.1 due to energy saving feature.

So yeah, I think @teddyastie 's idea is a good approach here. Try how your setup behaves during a workload.

[redakula]

@MajorP93

I spent quite some time on this when i first got an Arc card and all research i found points to Linux reporting the top of the PCIE chain only. The card has an internal bridge.

The only tool i found that reports the correct info is GPU-Z which is unfortunately windows only.

My A310 which also reports PCIE x 1 in linux reports the correct link speed in GPU-Z (It is in a X4 slot so that is why it is not showing the X8 that the card is...)

[john.c]

Is there anything mentioned in the motherboard or service manuals, about where if one PCIe slot is occupied another is running at reduced speed when occupied?

Something like this is in the manual for a Gigabyte B450 Aorus Elite (Rev 1.0) motherboard, for example. Can be in yours, somewhere written on one of its pages.