PCIe Pass-through lanes and lane performance

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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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.

JamesG

So I ordered some mini DP dummy plugs and took a look at this further today.

First thought...Just load Windows on the bare metal and see what the card does.

No problem. Loaded Win10, loaded the latest drivers, PCIe Gen4x8 lanes. Sweet. At least that works.

Put the XCP-ng drive back in, booted up, passed-through the GPU to a Win11 guest, updated the drivers, PCIe Gen1x1.

Ultimately I need to install a Linux distro with a really current kernel on the bare metal and see how it sees the GPU. That might be a good project for tomorrow.

I think this just comes down to a combination of too new, not well supported hardware with some quirks and somewhat dated code and support in XCP-ng (ReBAR support in guests for example).

@redakula Is that GPU-z screenshot from a Windows guest VM with the A310 passed through?

yannsionneau

Maybe let's ping the entire @Team-Hypervisor-Kernel here

JamesG

@yannsionneau I appreciate that. I will gladly provide access to the hardware if anybody in development wants to dig in and see what's going on.