PCI Nvidia GPU Passthrough boot delay
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If I understood correctly what I've been told, the "hand time" is in fact mapping (and remapping) the PCIe BAR.
It takes some time to be done, until we (ie the Xen community) got some patches merged in Xen, allowing to enjoy IOMMU superpages, which should turn all of this into a fraction of a second. I don't know the current status of this.
Regarding permissive thing, I don't know I have to ask around too.
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@omg said in PCI Nvidia GPU Passthrough enable permissive?:
It appears to be consistently 20 - 30 seconds per RTX Ampere GPU, about 20 - 25 seconds on Quadros and ~90 seconds on an A100.
What's worse on the A100, it seems the calls are made linear so say I pass through four A100s the wait time to boot will be 4x90s, not optimal.These are known, and yeah - they are not great. It's an issue in Xen where the IOMMU logic doesn't (yet) support superpage mappings, so time delay you're observing is the time taken to map, unmap, and remap the GPU's massive BAR using 4k pages. (It's Qemu taking action in response to the actions of the guest.)
The good news is that IOMMU superpage support is in progress upstream, and should turn this delay into milliseconds.
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Thank you both for the reply.
What's interesting is that Xen reports larger IOMMU page size support, not just 4k. So it's just lying to me that these are supported? :]
(XEN) [ 6.844448] Intel VT-d iommu 8 supported page sizes: 4kB, 2MB, 1GB (XEN) [ 6.852537] Intel VT-d iommu 7 supported page sizes: 4kB, 2MB, 1GB (XEN) [ 6.860614] Intel VT-d iommu 6 supported page sizes: 4kB, 2MB, 1GB (XEN) [ 6.868703] Intel VT-d iommu 5 supported page sizes: 4kB, 2MB, 1GB (XEN) [ 6.876790] Intel VT-d iommu 4 supported page sizes: 4kB, 2MB, 1GB (XEN) [ 6.884877] Intel VT-d iommu 3 supported page sizes: 4kB, 2MB, 1GB (XEN) [ 6.892963] Intel VT-d iommu 2 supported page sizes: 4kB, 2MB, 1GB (XEN) [ 6.901040] Intel VT-d iommu 1 supported page sizes: 4kB, 2MB, 1GB (XEN) [ 6.909131] Intel VT-d iommu 0 supported page sizes: 4kB, 2MB, 1GB (XEN) [ 6.917221] Intel VT-d iommu 9 supported page sizes: 4kB, 2MB, 1GB(XEN) [ 7.495448] HVM: HAP page sizes: 4kB, 2MB, 1GBI gather from your explanation I guess it won't matter to get permissive mode to work or not...Still wanted to try out of curiosity.
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Trying this on a guest without GPU drivers yields the same delay but no Write-back warning so complete proof that the delay is entirely due to the slow reading of the GPU's BAR. Not that I doubted anyone ;]
Already made complete sense previously since the A100 has such a large BAR in comparison to the others.
@ndyhhp can you point me to the work being done upstream around IOMMU superpage support?
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@ndyhhp said in PCI Nvidia GPU Passthrough boot delay:
The good news is that IOMMU superpage support is in progress upstream, and should turn this delay into milliseconds.
Assuming this is the work you’re referring to: https://lists.xenproject.org/archives/html/xen-devel/2022-01/msg00277.html
Any idea when this will be committed?
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@omg That is the work, but it needs rebasing over the XSA-400 work, so a v4 series is going to be needed at a minimum.
HAP is Xen's vendor-neutral name for Intel EPT or AMD NPT hardware support. We have had superpage support for many years here.
IOMMU pagetables can either be shared with EPT/NPT (reduces the memory overhead of running the VM), or split (required for AMD due to hardware incompatibilities, and also required to support migration of a VM with an IO devices).
When pagetables are shared, the HAP superpage support gives the IOMMU superpages too (because they're literally the same set of pagetables in memory). When pagetables are split, HAP gets superpages while the IOMMU logic currently uses small pages.
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T tomg has marked this topic as solved on
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@livierlambert said in PCI Nvidia GPU Passthrough boot delay:
If I understood correctly what I've been told, the "hand time" is in fact mapping (and remapping) the PCIe BAR.
It takes some time to be done, until we (ie the Xen community) got some patches merged in Xen, allowing to enjoy IOMMU superpages, which should turn all of this into a fraction of a second. I don't know the current status of this.
Regarding permissive thing, I don't know I have to ask around too.
Looks like this finally got committed last week!
https://github.com/xen-project/xen/commits/master/xen/drivers/passthrough
@livierlambert - any idea when this will make it into XCP-ng? :]
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You are right, but the code isn't into the right shape right now.
However, it's going indeed into the right direction! As soon it's possible to backport it (or to add it into a more recent release), we'll try to make it real
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@livierlambert any idea if Xen 4.17 will be coming to XCP-ng? :]
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Probably (ideally) before the end of 2024, but it's really hard to make any promises (it might be -at that point- an ever more recent version)
