VFIO MB/GPU Question

I have the ASUS ROG Strix X570-F Gaming Motherboard and it says that it has 2 x PCIe 4.0 x16 (x16 or dual x8).

First let me note that I’m running Fedora 34:

And I’m working on a VFIO build which has taken me a VERY long time due to the Varus and other complications here in Australia. If I’m running two PCIE 4.0 GPU’s, one for host, one for VFIO (looking glass) guest, would they run at full 4.0 bandwidth when one is in use while the other is in idle?

Does it matter if I run both PCIE 4.0 cards on 8x on both host and guest? How does this work?

Cool, good talk guys. Thanks for the valuable input, I have learned sooooo much.

Read if you care

I wouldn’t be asking here if I didn’t already know the answer elsewhere where it be google, other forums and chat clients. I’m also tech savvy, just new to the VFIO field, even re-reading multiple VFIO tutorials to understand the process of qemu and v-kvm + looking glass.

This really needed just a yes or no answer, and a brief explanation on how PCIE works in VFIO in terms of speed modes. But hey, if a mod see’s this and if no one cares to answer, your good to lock it, I apologize for wasting anyone’s time here.

Don’t be an ass on the internet. The forum is full of helpful and nice ppl.
What it is not is a professional help desk where you expect answers in a given time.
Chill out.


If your MB says this, then while both GPUs are plugged in, both slots will run at PCIe 4.0 x8. Does any of your GPUs need more than 16 GB/s? If not, you will be fine, and won’t loose performance due to bandwith limitations.

Assuming that will be true, and you don’t fallback on Gen3.

Interesting, all it took is to be an “ass”, I did wait for 2 days for an answer, any answer, I guess I was expecting too much here.

Thanks for your input, I guess I’ll test the cards on and off and see the limitations myself, was hoping for a more convenient function of lane switching but I guess that’s asking too much. My expectation was if the card has been pulled off when the system is booting, and once it was assigned in qemu, it had the full lane of x16.

Boohoo. This forum is not a ticket system with ppl employed to watch over it.
Change your attitude man.
/mute all

I feel like I don’t understand you. I mean, the MB says it will operate two occupied slots at Gen4 x8. So if you have anything plugged in both slots, both will run x8, not x16.
I’m outside my comfort zone anyway, sorry for not being able to help.

No worries mate, my expecations of L1T forums was from it’s conception, I haven’t been using it for a long time and everytime I posted it was answered in less than an hour or so, I guess times have changed. I’m also used to quick response outlets so in this case, I’m in the wrong and I apologize.

I’ll just test it myself and if it doesn’t work out for me I’ll try to manage a board which can support both PCIE 4.0 lanes at x16…my wallet will cry but it’s a sacrifice I’m willing take.

I don’t think there’s any x570 boards that have what your looking for, right in the spec, it says 1x16 or 2x8.

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Yup that is not really an option because mainstream cpu’s like Ryzen only have 16 pci-e,
lanes for pci-e slots.
If he want’s both cards at full 16x pci-e 4.0 speeds then he should go with Threadripper.
However even at 8x pci-e 4.0 there won’t be any bottlenecks regards to running gpu’s.
So there is nothing to be concerned about.

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