Lower pcie bandwidth?

Software & Operating Systems Looking Glass
1

I’ve been using looking glass for a while with a 3060ti passed through a gen4 16x slot and an old 750ti for the host in a gen3 4x slot in a b550 mobo. It’s just 1440p 75hz, so it’s been working great.

I just installed a usb pcie card to an empty 1x slot which dropped the 750ti to 1x also due to shared lanes. I can run the VM up to 2560x1080 60hz consistently, but once I set it to 2560x1440 looking glass drops down to a very glitchy ~45fps but still 75ups. I assume this is just due to the lower 1x bandwidth, and if I switch display inputs the native 3060ti output is still fine.

Do I need to ditch this usb card/get an x570 motherboard? Is there any way I can alleviate the bandwidth bottleneck? I seem to be pretty close to having enough bandwidth, but not quite.

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Yes, LG needs to shuffle the frames from the passthrough GPU to the other GPU for display.

Can’t you just use a USB hub?

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3

The latest looking glass has FSR upscaling, might tide you over until a new platform is released.

Other than that which mobo and usb card are you using? In theory the bandwidth is shared, if the USB card is idle the 750Ti should be able to use it.

If your mobo supports it, and the cpu m.2 slot is free, you could also try an m.2 to pcie riser.

4

If the GPU is reporting it’s negotiated a 1x link then this doesn’t hold true.

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If I could figure out how to properly pass a hub through, or even just a single usb port on my machine, I’d be happy with that. I’m using virt-manager and when I add a hub it doesn’t seem to actually pass devices connected to it.

I’m trying to use an ASIO capable usb guitar amp that doesn’t work properly if I simply pass it through as a device. It does work properly on the passed through pcie usb card.

It’s an Asus TUF B550 Plus. The usb card is just some generic thing, but whenever the extra 1x slot has anything connected it drops the 4x down to 1x.

I am using both m.2 slots already lol.

6

According to the manual for your motherboard, it should have the following BIOS option:

PCIEX16_2 4X-1X Switch
This item allows you to configure PCIEX16_2 operating mode.
[Auto] Auto detect and switch. PCIEX16_2 runs x4 mode by default. If a PCIE
device is installed on PCIEX1_1, PCIEX1_2 or PCIEX1_3, PCIEX16_2 will
run x1 mode.
[1X] Force PCIEX16_2 to run x1 mode.
[4X] Force PCIEX16_2 to run x4 mode

The operating mode options of PCIEX16_2 vary by motherboard models.

Can you verify if you have that setting in your BIOS, and if you do what happens if you set it to “4x”?

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I am able to force it to 4x, but all that does is totally disable the three 1x slots regardless of what’s connected to them.

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That’s unfortunate, I guess limiting lanes like that is cheaper on lower end boards.

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Another option would be to pass one of your USB controllers to the guest. This would give it all the ports on said controller, but most PCs these days have several USB controllers and enough ports to give one to the VM.

If you then need more ports, use a hub on the USB controller that needs more ports.

Personally this is how I do things.

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Yeah, I’ve been trying to avoid having to do that because Asus really cheaped out on the number of controllers they’re using. Exactly two. One for the four high speed USB 3.2 (or whatever it’s called now) and USB-c ports and one for literally all the rest. I’ll probably pass through those four ports in the meantime while I contemplate getting a motherboard that’s not so limited in its configuration lol.

11

Using the WiFi slot?

Also a thunderbolt header I haven’t looked into yet

Available for ~$20 depending on length (5cm is cheapest)

(I used an m.2 NVMe slot converter to M.2 wifi in my board because it had 4 NVMe slots but didn’t have any Key E for my wifi :laughing:)