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On 2020/5/28 17:09, Adrian Moreno wrote:
> Hi,
> 
> Currently VFIO passthrough on q35 is broken because of two connected issues:
> 
> - pci-rescan race condition: https://github.com/kata-containers/runtime/issues/2664
> - Pcie-root-port failing: https://github.com/kata-containers/runtime/issues/2678
> 
> So far, my understanding is that:
> 
> - The PCI hotplug/rescan race condition affects both bridge and root-port
> hotplugging and regardless of the device's properties
> - The rescan is kept because of the 5s delay shpc introduces when devices are
> hotplugged on a pcie-to-pci bridge
> 
> Am i missing something?
> 
> Based on that I would suggest:
> 
> 1 - Disable the rescan for pcie-root-port hotplugging as that kind does not
> incur in the 5s delay that justifies the rescan AFAIK.
> 
> 2 - Study a way of systematically avoiding the race on bridge hotplugging. For
> this we need further investigation. I personally have many questions:
>     - Can the 5s delay be avoided in some other way?
>     - If we are scanning the bus anyway, can we make shcp be manualy triggered? I
> see it has a "polling mode". Has that been investigated?
>     - What is the cost (in startup time, memory, etc) of having a high number of
> pcie-root-ports (to avoid the cost of an exact estimation)
> 
> 3 - Investigate whether we can fix this in the kernel. I have not looked into
> this but maybe shpc could detect the situation and "fail" gracefully not messing
> up the pci device.
Is it a known issue to the kernel community? If not, we should report to 
kernel maintainers first.

Thanks,
Tao

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