Re: [RFC] /dev/ioasid uAPI proposal

On Fri, 4 Jun 2021 09:19:50 +0000
"Tian, Kevin" [off-list ref] wrote:

quoted

From: Alex Williamson <redacted>
Sent: Friday, June 4, 2021 4:42 AM
  

quoted

'qemu --allow-no-snoop' makes more sense to me  

I'd be tempted to attach it to the -device vfio-pci option, it's
specific drivers for specific devices that are going to want this and
those devices may not be permanently attached to the VM.  But I see in
the other thread you're trying to optimize IOMMU page table sharing.

There's a usability question in either case though and I'm not sure how
to get around it other than QEMU or the kernel knowing a list of
devices (explicit IDs or vendor+class) to select per device defaults.
  

"-device vfio-pci" is a per-device option, which implies that the
no-snoop choice is given to the admin then no need to maintain 
a fixed device list in Qemu?

I think we want to look at where we put it to have the best default
user experience.  For example the QEMU vfio-pci device option could use
on/off/auto semantics where auto is the default and QEMU maintains a
list of IDs or vendor/class configurations where we've determined the
"optimal" auto configuration.  Management tools could provide an
override, but we're imposing some pretty technical requirements for a
management tool to be able to come up with good per device defaults.
Seems like we should consolidate that technical decision in one place.
Thanks,

Alex