[vfio-users] FLR supporting serial chips?

[Alex Williamson]

On Mon, 7 Aug 2017 17:52:04 -0400
"Taiidan at gmx.com" <Taiidan at gmx.com> wrote:

> I want to add physical serial ports to a VM but I can't find any FLR 
> supporting serial cards (I have looked at spec sheets for the chips no luck)
> 
> Ideally there would be a multi port card (say 4 or so) where every port 
> can be separately assigned.
> 
> Anyone have any info? - Thanks

I think you're headed in the wrong direction, serial ports and PCI
device assignment can be done, but it's not a great match.  For
starters, serial ports don't do DMA, so the whole IOMMU thing is a bit
overkill.  You're also not likely to find a serial ASIC designed within
the last 20yrs, so things like ACS and FLR are mostly out of the
question, besides the device state of a serial port is pretty minimal
and a device specific reset could probably be made to handle it, but
I'm not sure it's even necessary.  Performance is also likely to be not
much, if any, better than just serial passthrough because the register
set of a serial device doesn't fill a 4K page or may be implemented as
I/O port rather than MMIO, so you'll probably trap to QEMU for every
access anyway unless you can get 4k alignment on the MMIO BARs.  USB
serial passthrough might be a better option, but FLR seems
exceptionally unlikely. Thanks,

Alex