Re: [PATCH V2] vhost: correctly turn on VIRTIO_F_IOMMU_PLATFORM

[Halil Pasic]

On Wed, 26 Feb 2020 04:53:33 -0500
"Michael S. Tsirkin" <address@hidden> wrote:

> On Wed, Feb 26, 2020 at 05:43:57PM +0800, Jason Wang wrote:
> > We turn on device IOTLB via VIRTIO_F_IOMMU_PLATFORM unconditionally on
> > platform without IOMMU support. This can lead unnecessary IOTLB
> > transactions which will damage the performance.
> > 
> > Fixing this by check whether the device is backed by IOMMU and disable
> > device IOTLB.
> > 
> > Reported-by: Halil Pasic <address@hidden>
> > Fixes: c471ad0e9bd46 ("vhost_net: device IOTLB support")
> 
> Well it's just an optimization, isn't it?
> I don't think it's justified to push this to everyone using
> vhost with IOTLB, is it?

IMHO we need this for everyone using vhost! For instance vhost-vsock
currently does not work with iommu_platform=on, because unlike vhost-net
vhost does not offer F_ACCESS_PLATFORM, so set features IOCTL fails. 

> If you disagree, could you comment a bit on which configurations where tested?
> 
> > Cc: address@hidden
> > Signed-off-by: Jason Wang <address@hidden>
> 
> Halil could you test this pls? Does this help your performance issue?
> 

I'm pretty sure it does, but I will re-test. The previous version where
it was done virtio-net certainly did.

Regards,
Halil