[vfio-users] about vfio interrupt performance

[Alex Williamson]

On Tue, 18 Jun 2019 19:43:39 +0800
James <smilingjames at gmail.com> wrote:

> Hi Alex:
> 
> Many thanks for your detailed feedback and great helps!
> 
> 1, yes, make our drive into upstream will also solve this problem :)
> 
> 2, got it, checking device's some status register mapped via
> vfio persistently will be a better solution compare with eventfd if
> interrupt rate is high.
> 
> 3, "1, Some device’s extend configuration space will have problem when
> accessing by random."
> It means I remeber some guy reported that when they try to access extend
> configuration space via vfio framwork, sometimes they'll get access error,
> not all device have this problem(it only happen to extend configuration
> space), it happen rarely.
> I forget the link of this issue, not sure if you have some comments to this
> kind of issue, so sorry to mislead you and waster your time..

I don't recall issues here.  Config space sometimes returns error due
to failed bus reset at the device or root port (typically AMD platforms
or GPUs), otherwise there are VM machine restrictions on extended
config space (conventional vs express machines), but I don't know of
any persistent issues beyond that.

> 4, and to "2, When try to access the device’s space which in the same iommu
> groups at the same time, it will trigger issue by random"
> You mean if we can not sperate the device into different iommu groups, we'd
> better not access two device which in the same groups at the same time.

An IOMMU group is the minimum unit of ownership for a user.  A user can
make use of all endpoints within an IOMMU group, but devices within the
group cannot be split between multiple users or between user and host
use cases.  The IOMMU group represents the smallest unit of isolation.
Thanks,

Alex