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Re: [Qemu-devel] About virtio device hotplug in Q35! 【外域邮件.谨慎查阅】

From: Alex Williamson
Subject: Re: [Qemu-devel] About virtio device hotplug in Q35! 【外域邮件.谨慎查阅】
Date: Tue, 22 Aug 2017 10:56:59 -0600

On Tue, 22 Aug 2017 15:04:55 +0800
Bob Chen <address@hidden> wrote:

> Hi,
> I got a spec from Nvidia which illustrates how to enable GPU p2p in
> virtualization environment. (See attached)

Neat, looks like we should implement a new QEMU vfio-pci option,
something like nvidia-gpudirect-p2p-id=.  I don't think I'd want to
code the policy of where to enable it into QEMU or the kernel, so we'd
push it up to management layers or users to decide.
> The key is to append the legacy pci capabilities list when setting up the
> hypervisor, with a Nvidia customized capability config.
> I added some hack in hw/vfio/pci.c and managed to implement that.
> Then I found the GPU was able to recognize its peer, and the latency has
> dropped. ✅
> However the bandwidth didn't improve, but decreased instead. ❌
> Any suggestions?

What's the VM topology?  I've found that in a Q35 configuration with
GPUs downstream of an emulated root port, the NVIDIA driver in the
guest will downshift the physical link rate to 2.5GT/s and never
increase it back to 8GT/s.  I believe this is because the virtual
downstream port only advertises Gen1 link speeds.  If the GPUs are on
the root complex (ie. pcie.0) the physical link will run at 2.5GT/s
when the GPU is idle and upshift to 8GT/s under load.  This also
happens if the GPU is exposed in a conventional PCI topology to the
VM.  Another interesting data point is that an older Kepler GRID card
does not have this issue, dynamically shifting the link speed under
load regardless of the VM PCI/e topology, while a new M60 using the
same driver experiences this problem.  I've filed a bug with NVIDIA as
this seems to be a regression, but it appears (untested) that the
hypervisor should take the approach of exposing full, up-to-date PCIe
link capabilities and report a link status matching the downstream

I'd suggest during your testing, watch lspci info for the GPU from the
host, noting the behavior of LnkSta (Link Status) to check if the
devices gets stuck at 2.5GT/s in your VM configuration and adjust the
topology until it works, likely placing the GPUs on pcie.0 for a Q35
based machine.  Thanks,


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